Six patients with Parkinson's disease, six patients with essential or intention tremor, and nine controls were tested on a step-tracking task using a joystick control and oscilloscope display. Tremor subjects resembled controls in making an initial ballistic movement followed by (defective) corrections, and took longer than the controls to reach the target with small amplitude jumps, but not with larger ones. The reverse was true for parkinsonian subjects, who acquired the target with slow corrective movements only. This suggests that two kinds of movement available to normal people are selectively impaired in these disorders; ballistic movements in Parkinson's disease and small amplitude corrective movements in the other disorders.