Injuries to the ulnar collateral ligament frequently occur in throwing athletes because of large, repetitive valgus stresses to the elbow during the cocking and acceleration phases of throwing. Identification of injury to this ligament is important in evaluating the throwing elbow. The purpose of this study was to determine whether differences in medial elbow laxity exist between the dominant and nondominant extremities in uninjured baseball pitchers. Forty uninjured professional baseball pitchers were tested bilaterally with a Telos GA-IIE stress radiography device. Joint space width between the trochlea of the humerus and the coronoid process of the ulna was measured on anteroposterior radiographs obtained with no stress applied and with a 15-daN valgus stress. Results showed significant differences between the medial joint space opening of the dominant and nondominant elbows with no stress applied. With stress, the dominant elbow opened 1.20 +/- 0.97 mm, while the nondominant elbow opened 0.88 +/- 0.55 mm. A significantly greater difference in medial joint space opening between the stressed and unstressed elbows was measured in the dominant elbow compared with the nondominant elbow (0.32 +/- 0.42 mm). This study identifies increased medial elbow laxity in the dominant arm in uninjured pitchers.