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Background: Physical risk factors for osteochondritis dissecans (OCD) of the humeral capitellum in young baseball players have not been fully elucidated. We aimed to identify the risk factors for capitellar OCD in baseball players aged 8-14 years. Methods: Between December 2018 and December 2019, young baseball players were recruited from 8 regional baseball leagues. Ultrasonography and physical assessments were performed preseason and at the end of the study period. Bilateral passive ranges of motion (ROM) of horizontal adduction of the shoulders, internal rotation (IR) of the hips, and the thoracic kyphosis angle were measured. 1-year follow-ups were scheduled to determine the occurrence of OCD. Players with OCD were categorized into an OCD group; those without OCD and any elbow pain for one year were categorized into a non-injured group. The players’ baseline data (age, sex, position in baseball, and Rohrer's Index) were analyzed using univariate analyses. Their physical parameters were analyzed using two-way analysis of variance with repeated measures to investigate OCD-related risk factors. Results: In total, 3651 baseball players attended the 1-year follow-up. Of these, 71 (1.9%) players had OCD of the humeral capitellum. In the OCD group, a significant association was found at baseline and at the end of the study period between a higher Rohrer index at the baseline and a smaller hip IR ROM on the nondominant side. Conclusions: Loss of hip IR ROM on the nondominant side is a newly discovered risk factor related to physical function in the development of OCD.