The purpose of this investigation was to establish a data base regarding the isokinetic muscular performance characteristics of the abductor and adductor muscles of professional baseball pitchers. Eighty-three healthy professional baseball pitchers (mean age, 22.6 years; mean weight, 199 pounds) were evaluated by use of a Biodex isokinetic dynamometer. Isokinetic tests were performed concentrically at 180 and 300 deg/sec for both the throwing and nonthrowing shoulders. The testing protocol and test repetitions were standardized for each subject. Statistical analysis was performed using a paired t-test. Determination of the correlation coefficient was made at the P < 0.05 level of significance. Test results for bilateral comparisons of mean peak torque for the throwing and nonthrowing shoulders demonstrated a significant difference in adductor values between the dominant and nondominant shoulders at both test speeds. There were no significant differences between extremities for the shoulder abductor muscles. The abductor-to-adductor muscle ratios between the throwing and nonthrowing shoulders were also statistically significant at both test speeds. Throwing arm values were 82.5% at 180 deg/sec and 93.8% at 300 deg/sec compared with only 66.0% and 70.3%, respectively, for the nonthrowing shoulders.