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: The objective of this study was to compare the humeral retroversion angles HRA between baseball players, including children, and those without a history of playing baseball, clarify the characteristics of the HRA in baseball players, and to determine whether or not chronological changes of the HRA are affected by a throwing motion. We studied 32 young baseball players Group A , 10 elementary and junior high school students who had never played competitive overhead throwing sports Group B , 65 adult baseball players who had been playing baseball since childhood in a little league or boy s baseball team Group C , and 11 adults who had never played competitive overhead throwing sports such as baseball or handball Group D . Computed tomography of both humeri in these subjects was taken with a 5 mm slice thickness. For the measurement of HRA, slices from the center of the humeral head and slices from the humeral epicondyle were examined. In baseball players, the mean HRA on the throwing side was larger than that on the non-throwing side, regardless of age and carrier. The HRAs of the elementary and junior high school baseball players as well as those of adult baseball players were larger on the throwing side. The HRA of the throwing side was signi cantly greater than that of the non-throwing side in both groups of baseball players. Furthermore, the mean HRA on the throwing side of young baseball players was signi cantly larger than that of adult baseball players, suggesting that the adaptive bony change of the humerus was caused by throwing stress and might occur in the early formative years of a player s career.