Investigation of motor function in a group of 17 patients with Huntington's disease reveals that, in addition to the chorea that many patients exhibit, defects in voluntary motor performance also are evident. Fast simple wrist flexion movements to 15 degrees or 60 degrees were slower, and individual movements showed greater variability than seen in normal subjects. This bradykinesia was most pronounced in those patients who were akinetic and rigid, but also was seen in those with chorea alone; bradykinesia was independent of the drug treatment that the patients were receiving (and was therefore not due to drug-induced parkinsonism). The electromyographic activity of the agonist muscles during such simple but slow movement differed from that seen in Parkinson's disease. The performance of complex movements revealed further deficits. Some patients were unable to combine two movements in a simultaneous or sequential movement task of squeezing the hand and flexing the elbow. Those who could perform these complex movements exhibited slowing of the velocity of the movement and prolongation of the interval between movements. These abnormalities were present in patients with chorea who were not taking neuroleptic drugs. It is argued that they represent an abnormality of motor programming of complex movements, over and above the defect in executing simple movements. The long latency stretch reflexes in wrist flexor muscles and flexor pollicis longus were reduced or absent, but this did not correlate with changes in motor performance, or with the reduced size of the early components of cortical sensory evoked potentials. Bradykinesia is thus shown to be an integral component of the motor disorder of Huntington's disease, in addition to the chorea. The coexistence of bradykinesia and chorea in this illness is compatible with current theories of the role of the basal ganglia in the control of movement.