This study attempted to establish a reliable test protocol to measure the strength of muscles involved in elbow and knee flexion and extension for mentally retarded (MR) adults. Nineteen mildly MR adults performed strength tests on a Cybex 340 isokinetic dynamometer on two separate days; 12 subjects performed a third trial of knee flexion and extension tests. For the 19 subjects, there were no significant differences in strength measurements--except for peak torque of knee flexion--between the first two test days. There were relatively high correlation coefficients when test parameters were compared for both test days, and best efforts occurred almost as often on test day two as on test day one. For the 12 subjects tested on three test days, there were no significant differences for test parameters among the three test days; however, 25% of best efforts took place on test day three. These findings indicate that when measuring the isokinetic strength of mildly MR individuals, tests should be performed at least two, optimally three, separate times to ensure reliability. The test protocol would be reliable when measuring knee and elbow flexion and extension strength for job assessment or clinical or rehabilitation purposes.