Objective: Main objective of this study was to determine and describe shoulder injury percentages and its indcidence in a strictly chosen group of vaterpolo players before and after the second part of the season. In addition to revealing statistical data about injury occurence in vaterpolo players the goal was to educate them on how to prevent and rehabilitate shoulder injuries so they can fully return to professional sport or continue their ongoing training program. Methods: In this cohort study there was 37 participants included as a part of one homogenous group. Measurements were taken at the start and at the end of waterpolo half-season. Data was collected 1.2.2021. and 1.6.2021. Constant Murley Score questionaire was used for grading the degree of injuries and pain. The questionaire consisted of range of motion measurements, strength measurements and pain measurements affecting the quality of life of the subjects.The tests were divided in two parts: the subjective and the objective part of the questionaire. Subjective part is filled by the player and objective part is filled by the examiner.Measurements at the start and at the end were statistically processed. Results: Statistical analysis with a confidence interval of 95% shows that a statistically relevant difference was measured in the shoulder internal rotation component (p=0.0434). Shoulder strength measurements showed a dramatic decrease (p=0.0002). Other parameters (pain within 24 hours, the effects of pain on daily life/activities, shoulder abduction,flexion and external rotation of the shoulder) did not show a statistically relevant difference when analyzed by the CMS questionnaire. Conclusion: Shoulder internal rotation is vastly limited in sportspersons and elucidates a deficit which needs to be addressed in the described prevention program. Shoulder strength in waterpolo players declines by the end of the half-season and is one of the indicators of shoulder damage and possible shoulder injury onset. This problem needs to be addressed via the described prevention program. In similar papers, shoulder measurements show the external rotation component as one of the focal points that need to be optimized in the future, since mild reduction in the range of motion is indicative of muscle imbalances of the shoulder girdle, especially scapular dyskinesis.