Background: Few researchers have examined the different contributions of flexor-pronator muscles to valgus stress in high school baseball pitchers with and without elbow symptoms. This study used ultrasonography to assess these muscles' dynamic contributions to elbow valgus joint stability in high school pitchers. Methods: A self-administered questionnaire on throwing-related elbow joint pain sustained during the prior year was completed by 89 high school baseball pitchers. Gravity stress ultrasonographic elbow examinations with 30° of flexion were done before and after isometric contraction of the flexor-pronator muscles. For study participants with and without a history of elbow symptoms, we compared differences of ulnohumeral joint space without gravity stress and isometric contraction of the flexor-pronator muscles and with gravity stress only and with isometric contraction of the flexor-pronator muscles under gravity stress. Results: For each pitcher, the ulnohumeral joint space in the condition with valgus stress was significantly larger than in the condition without valgus stress. Also, the ulnohumeral joint space in the condition with valgus stress was significantly larger than in the condition with valgus stress and flexor-pronator isometric muscle contraction. Participants with and without elbow symptom history showed no differences of ulnohumeral joint space in the unloaded and loaded flexor-pronator muscle contracted conditions. Conclusion: Ultrasonographic assessment demonstrated that flexor-pronator muscles contribute to elbow valgus stability. No difference was found in the flexor-pronator muscle contribution in high school baseball pitchers with and without elbow symptom history.