Patients with cerebellar ataxia often show swing of the upper arms or the elbows in forearm pronation-supination test (FPS test). Eleven healthy volunteers and sixteen patients with cerebellar ataxia were examined. We observed FPS test of the subject with their shoulder and elbow joints being flexed at a right angle respectively in sitting position, and recorded them by VTR. We noticed following four points in FPS movement: 1) velocity and regularity, 2) amplitude, 3) finger posture, 4) state of elbow fixation. We recorded surface EMG of the upper arm-fixing muscles (deltoid, pectoralis major, infraspinatus) and the forearm-moving muscles (biceps brachii, pronator teres). In controls, FPS movements were regular and speedy. The upper arm was so well fixed that the elbow did not swing. Continuous discharges were recorded from upper arm-fixing muscles when the upper arm was lifting. Reciprocal grouping discharges were recorded during FPS movements from upper arm-fixing muscles so that they were suppressing the swing of the elbow. In ataxic patients, FPS movements were dysrhythmic and slow. The elbow swung during FPS test. We observed internal rotation and abduction of the shoulder when forearm pronated, and outer rotation and adduction of the shoulder when forearm supinated. Reciprocal grouping discharges that were reverse of normal ones were recorded from upper arm-fixing muscles so that they were aggravating the swing of the elbow. Synergia is an ability to accomplish the various muscle contractions coordinately when making up a certain action. Asynergia is one of the characteristic features of disturbance of the association of muscle contractions of the upper arm and the forearm. FPS test is easy and useful to detect asynergia of the upper extremity in cerebellar ataxia.