The motor performance of seven patients with Parkinson's disease and seven control subjects was tested in a choice reaction aiming task. The subjects were instructed to aim as fast and as accurately as possible to a light stimulus, which defined one of eight possible target positions. In order to reach the targets, elbow flexions had to be combined with forearm supinations or with forearm pronations. For single-joint movements, forearm supinations or pronations were executed faster than the long elbow flexions in both groups. In the double-joint movements of the control group, the flexion movement times (flex.MTs) and the supination movement times (sup.MTs) or pronation movement times (pron.MTs) were similar to the MTs of the corresponding single-joint movements. MTs of parkinsonian patients were significantly longer than those of control subjects. MTs were most increased in the forearm supination and forearm pronation of double-joint movements. In contrast to the controls, sup.MT and pron.MT were significantly increased in double-joint movements as compared to single-joint movements. The variations in the flex.MT and sup.MT of the double-joint movements neither correlated for a control subject nor for a parkinsonian patient. For controls, the independent MTs in double-joint movements cannot be explained by minimal principles (minimum energy, minimal torque change), but suggest that two separate motor programs are superimposed. In Parkinson's disease, there seems to be a deficit in superimposing two separate motor programs.