The test-retest reliability of isokinetic muscle strength testing was determined in 45- to 78-year-old men (N = 71; mean age = 60.2 years) and women (N = 107; mean age = 60.0 years). The dynamic muscle strength of the knee and elbow extensor (KE, EE) and flexor (KF, EF) muscle groups was measured at slow (60 degrees/sec) and fast (K = 240 degrees/sec; E = 180 degrees/sec) angular velocities using a Cybex II isokinetic dynamometer on two occasions seven to ten days apart. The mean peak torque in the second test was significantly higher (p < 0.001) in all muscle groups and both angular velocities tested in both sexes. The angle at which peak torque occurred did not change from the first to the second test except in the KF of females tested at 60 degrees/sec. Pearson's correlation coefficients (CCs; p < 0.01) in males ranged from .68 to .77 in the knee and from .71 to .84 in the elbow. In females, the CCs (p < 0.01) ranged from .58 to .74 in the knee and from .67 to .78 in the elbow. Thus in a large number of older men and women at least two tests may be necessary to determine isokinetic peak torque.