We investigated the execution of two-dimensional, visually guided hand movements in patients with Parkinson's disease (PD) and in control subjects. Testing involved moving an unseen handle along a straight line or a sinusoidal path over a horizontally placed digitizing tablet. The path was displayed on a computer screen together with a pointer that represented the handle's location. The size of all video displays corresponded, with a 1:1 ratio, to the actual movements. We used the following tests. (1) Tracing, in which each subject was asked to move the pointer over the entire path, from left to right, with no speed requirements. (2) Tracking along a displayed path: a small circle moved along the path at a speed of 4, 7, or 10 mm/sec. The subject was asked to maintain the pointer inside the circle throughout its movement. When missed, the circle stopped moving until the pointer was again brought inside it. (3) Tracking along an unseen path: same as (2), except that the path was not displayed. Analysis of movement accuracy and of movement kinematics revealed that the patient's ability to control the direction of hand movement was impaired and that this impairment was evident in tracing as well as in tracking, was greater for sinusoidal than for straight-movement paths, and was independent of movement speed.