Waterpolo is a contact sport which also provides a shooting and a swimming phase and this aspect implies a major stress on the shoulder (21). It is assumed that in elite athletes repeating technical movements induces structural alterations of the stabilizing components of the shoulder, as already observed in studies performed on other overhead athletes (1,6). Studies have been performed related to the shooting mechanics of the baseball pitchers and the relative injury risk from shoulder overuse (3,6). The throwing biomechanics in waterpolo is similar to baseball but there are important differences: the dimensions and weight of the ball, the lack of a solid base during throwing phase. These characteristics influence the angular acceleration of the elbow and cause a decrease of the throwing speed (22). In literature, there are few data regarding changes of the rotator cuff in waterpolo (23). Previous authors have demonstrated that US and MRI have comparable and high accuracy for detecting full- and partial thickness tears in rotator cuff (24–26). MRI gives more information about muscle degeneration, the under acromion area and other coexisting pathologic process. We chose ultrasound because it is dynamic, less invasive and more sensitive than MRI in assessment of rotator cuff pre-insetional tendinopathy. Other studies carried out in volleyball, basketball, baseball and swimmers used US to evaluate the rotator cuff tendinopathy (1,24–27).The international protocol proposed by European Society of Musculoskeletal Radiology (ESMR) was adopted because considered as the most detailed and effective protocol for analysis and the comparison with other overhead-throwing studies (17). 61% of patients had a higher degeneration of the dominant supraspinatus compared with non-dominant (P< .05). In the 87% of athletes, the location of the tendon degeneration was anterior, lateral and pre-insertional, like in baseball and volley ball players (P< .05). All were outfield players. In the two goal-keepers the degeneration was bilateral, symmetrical and in a different position: posterior and medial. We suppose that this difference is due to the activities performed by goalkeepers that are not the same of outfield players: the goalkeeper makes rarely high energy shots on goal, but often performs slow passages at variable distance not exceeding 20 meters. During the defensive phases the upper limbs of the goalkeeper are both raised to defend the goal area or move simultaneously underwater, increasing the leg thrust to reach the extreme limits of the goal. In the sessions of swimming, unlike outfield players who practice predominantly freestyle, the goalkeeper performs predominantly butterfly. Like in other studies, no statistically significant difference in tendon diameter was found between dominant and non-dominant supraspinatus (2,28,29){Girometti, 2006 #15}{Ocguder, 2010 #27}{Wang, 2005 #32}. The more stressed structure in the waterpolo throwing technique is the supraspinatus tendon, like in other overhead throwing disciplines (1,5,6,8,29). Nine athletes (39%) had bilateral grade ≥1 supraspinatus degeneration because also non-dominant shoulder is stressed during swimming phases (23). The statistical analysis evidenced that among the variables studied, structural degeneration is the most influential to define the difference between the tendons of the shoulder. Ocguder et al. evidenced that, in overhead sports, supraspinatus tendon is overloaded, with its bursa (28). In our study, the supraspinatus alteration is associated with bursa in only 30% of cases and without a relevance in dominant or non-dominant arm. Subacromial bursa is low stressed in our study. We suppose that this difference is due to the time when we analyzed the athletes, at the beginning of the season. This difference into overhead disciplines is probably due also to a different shoot dynamic, heavy and measures of the ball, friction with the water, absence of fixed support. This induces a displacement of the vector sum of forces at the level of stabilizing structures of the shoulder, resulting in an increased stress at these level (22). The subscapularis tendon is less stressed in overhead sports, like described in other works in this study, therefore we analyzed the tendon echotexture and not the diameter (1,2,4,15,27). Only eight (35%) of the evaluated athletes had a tendon degeneration ≥1 at the dominant subscaularis. No difference of subscapularis degeneration between the two limbs confirms a predominant involvement of this tendon in swimming and less in the throwing phase (6). The LHB in the water polo player, unlike what is observed in the other overhead throwing disciplines, is very stressed (4,30). In the 96% of the cases a degeneration has been observed, 91% bilaterally. The area of the degeneration was for every athlete the same, where the tendon goes into the biceps pulley (P<.05). It can be assumed that this tendon is involved especially during the swimming phases, as seen in the study of Rodeo et.al in swimmers, who noted biceps tendinosis in 72% of the analyzed shoulders (6,31). Previous studies have shown that the severity of the tendon degeneration shoulder would correlate with the number of trainings per week, career years, the role and of course with any work activity that the athlete engages outside the pool. In this study, as in many others carried out on athletes, ages and years of activity were not influential (32). Ultrasound examination was completed for each patient with power doppler: previous studies, carried out mainly on Achilles and patellar tendons in marathon runners, footballers and volleyball players, have shown a variation of the local tendon vascularity associated with the increase of activity and have identified a data correlation with risk of injury during the course of the season. PD was defined as a predictive method for future pain and injuries (33–35){Gisslen, 2005 #16}{Hirschmuller, 2012 #17}{Richards, 2010 #28}. At the beginning of the season PD has shown low percentage of vascularity. Our next goal is to assess new PD evaluation at the end of the season and correlate the vascularity status with symptoms. Our hypothesis is that PD can have a predictive value of shoulder pain also for rotator cuff tendons. Strength points of our study are the careful high level water polo player selection: asymptomatic, no previous surgery or accident, no intra-perirticular infiltration. The US was performed at the beginning of the season, when tendon stress was low. The radiologist had 30 years of experience in shoulder ultrasound examination. It has been a single-blind evaluation: dominant arm, age, position, career years of the athletes were unknown to the radiologist. The study limits are the low number of chosen athletes: in Lombardy there are three Italian Premier League teams, composed by average 13-15 athletes in each team. The goal-keeper number is very low compared with movement players: a waterpolo team is composed by fifteen athletes, with only two goal-keepers. Left-handed players number is lower than right-handed: this aspect characterize also daily life. In a water polo team, usually there are one or two left-handed athletes.