Background: The overhead throwing athlete has unique range of motion characteristics of the shoulder and elbow. Numerous theories exist to explain these characteristics; however, the precise cause is not known. Although it is accepted that range of motion is altered, the acute effect of baseball pitching on shoulder and elbow range of motion has not been established. Hypothesis: There will be a reduction in passive range of motion immediately after baseball pitching. Study Design: Controlled laboratory study. Methods: Sixty-seven asymptomatic male professional baseball pitchers participated in the study. Passive range of motion measurements were recorded using a customized bubble goniometer for shoulder external rotation, shoulder internal rotation, total shoulder rotational motion, elbow flexion, and elbow extension on the dominant and nondominant arms. Testing was performed on the first day of spring training. Measurements were taken before, immediately after, and 24 hours after pitching. Results: A significant decrease in shoulder internal rotation (-9.5°), total motion (-10.7°), and elbow extension (-3.2°) occurred immediately after baseball pitching in the dominant shoulder (P <.001). These changes continued to exist 24 hours after pitching. No differences were noted on the nondominant side. Conclusion: Passive range of motion is significantly decreased immediately after baseball pitching. This decrease in range of motion continues to be present 24 hours after throwing. High levels of eccentric muscle activity have previously been observed in the shoulder external rotators and elbow flexors during pitching. These eccentric muscle contractions may contribute to acute musculotendinous adaptations and altered range of motion. The results of this study may suggest a newly defined mechanism to range of motion adaptations in the overhead throwing athlete resulting from acute musculoskeletal adaptations, in addition to potential osseous and capsular adaptations. © 2008 American Orthopaedic Society for Sports Medicine.