The purpose of this study was to describe the patterns of postural adjustments during induced body sway in healthy subjects and hemiplegic patients. Electromyograms were taken with surface electrodes from four lower extremity muscles that control ankle, knee, and hip motions. During postural adjustments in 10 healthy subjects, the ankle muscle and the complementary knee muscle contracted in fixed proportion. The first muscle adjustment, acting to stabilize the induced sway, occurred 95 to 118 msec after the onset of the perturbation. The sequence of muscle activation began at the ankle and proceeded proximally. Postural adjustments varied highly in 10 patients with clinically diagnosed right hemiplegia resulting from cerebrovascular accidents. The most frequent abnormalities included 1) synchronous contraction of several or all lower extremity muscles tested, 2) inconsistent patterns of muscle activation throughout each subject's trials, 3) longer, more varied response latencies than with healthy subjects, and 4) distorted sequence of muscle activation.