The amount of torsion in the humerus is determined by both genetic and activity-related factors, and affects the external rotation range of motion available at the shoulder. Previous research has shown athletes participating in throwing sports to have a greater amount of humeral retrotorsion in their dominant arm. The purpose of this study was to investigate the predictive ability of both the genetic and activity-related aspects of humeral torsion regarding throwing-related injury. The amount of humeral torsion in both arms of 35 high level adolescent male baseballers (mean age 16.6 years+/-0.6 years) was measured at study commencement. Significantly increased humeral retrotorsion in the dominant arm compared to the non-dominant arm was found (p=0.04). These athletes were followed for a period of 30 months, and any injury to their throwing arm which resulted in missing either a game or practice was recorded. ROC curve analysis was used to determine the predictive ability of humeral torsion with respect to the occurrence of injury to the throwing arm. Of the 35 athletes, 19 suffered a throwing arm injury. AUC values derived from ROC analysis showed humeral torsion in the non-dominant arm (AUC: 0.679, 95% CI: 0.502-0.857), as well as the average of the humeral torsion in both arms (0.692, 0.512-0.873), to be predictive of injury. Torsion in the dominant arm was not a significant throwing arm injury predictor. Thus non-dominant arm humeral torsion (the genetic contribution) was found to be the predictor of throwing arm injury.