Background: Baseball players exhibit a more posteriorly oriented humeral head or humeral retrotorsion (HRT) in the dominant arm, likely representing an adaptive response to the stress of throwing. This adaptation is thought to occur while skeletally immature, however there is limited research detailing how throwing while young influences the development HRT. In addition, it is currently unclear how this changing osseous orientation influences shoulder motion within young athletes. Purpose: To determine the influence of throwing and age on the development of asymmetry in HRT and shoulder range of motion (ROM); and analyze the relationship between HRT and ROM. Study Design: Cross-sectional age matched study Methods: Healthy athletes (8-14 years-old) were categorized into two groups based upon sports participation; throwing group (n=85) and non-throwing group (n=68). Bilateral measurements of HRT, shoulder external (ER), internal rotation (IR) and total range of motion (TROM) at 90° were performed using diagnostic ultrasound and digital inclinometer. A two-way analysis of variance was performed with throwing status (yes/no) and age group (youth (8-10.5), junior (10.51-12) and senior (12.01-13.99)) as primary factors. Dependent variables were asymmetry (dominant-non-dominant) in HRT,ER, IR and TROM. The relationship between ROM and HRT was analyzed using Pearson correlation coefficients. Results: Throwing athletes demonstrated a larger degree of HRT on the dominant side, resulting in greater asymmetry (8.7° versus 4.6°). Throwing athletes demonstrated a gain of ER (5.2°), a loss of IR (6.0°) and no change in TROM when compared to the non-dominant shoulder. Pairwise comparisons identified altered HRT and shoulder ROM in all age groups of throwers. A significant but weak relationship between HRT and shoulder ROM existed. Conclusion: Throwing causes adaptive changes in HRT and shoulder ROM in youth baseball players at a very young age. Other factors in addition to HRT influence shoulder motion within this population. Clinical Relevance: In baseball players, an altered arc of motion can be expected at a young age. This adaptation is in part due to changes in osseous structures, however a larger component of change is likely due to other factors.