Purpose <p>Ulnar neuropathy is a common condition among athletes who perform repetitive throwing motions. Dynamic displacement of the ulnar nerve during elbow flexion may contribute to medial elbow pain and impaired finger function. However, the reported prevalence of ulnar nerve displacement varies considerably, and differences in postural evaluation may contribute to these discrepancies. Therefore, this study aimed to determine whether shoulder abduction angle influences the prevalence of dynamic ulnar nerve displacement observed during ultrasonographic assessment in high school baseball players.</p> Methods <p>This cross-sectional study included 51 male high school baseball players from a single team. Dynamic ultrasonographic assessment of both elbows was performed at shoulder abduction angles of 30° and 100° to evaluate ulnar nerve displacement. Displacement during elbow flexion was classified into three categories: non-dislocation, subluxation, and dislocation.</p> Results <p>At 30° shoulder abduction, ulnar nerve displacement occurred in 35/50 (70.0%) throwing-side elbows and 26/50 (52.0%) non-throwing-side elbows. At 100° shoulder abduction, it occurred in 38/50 (76.0%) throwing-side elbows and 27/50 (54.0%) non-throwing-side elbows. The incidence rates were significantly higher on the throwing side than on the non-throwing side under both conditions (<i>p</i> = 0.001). Increasing shoulder abduction from 30° to 100° resulted in only a slight increase in the prevalence of ulnar nerve displacement, from 70.0% to 76.0% on the throwing side and from 52.0% to 54.0% on the non-throwing side; however, neither change was statistically significant (<i>p</i> = 0.69).</p> Conclusion <p>In high school baseball players, the prevalence of ulnar nerve displacement did not differ significantly between 30° and 100° of shoulder abduction. This finding suggests that shoulder abduction alone may have a limited influence on ulnar nerve dynamics.</p>