Visuomotor performance is known to be disturbed in patients who have advanced Parkinson's disease (PD). The degree of impairment in the early stages of PD was investigated in both the symptomatic and asymptomatic hand of patients with unilateral disease. We examined the visuomotor performance of 10 early unilateral parkinsonian patients and 15 control subjects by using several tracing and tracking tasks that were performed with unseen hands, in which feedback was given through a screen cursor. Significant impairment in control of movement direction was found in tracing of screen paths but not in tracking. Significant slowing of movement was found in target tracking, whereas tracing velocity was normal. Although all patients were judged by standard clinical criteria to be unilaterally affected, visuomoter functions were found to be impaired equally in both hands. Visuomotor control of movement direction and movement velocity is performed independently. Performance along both control dimensions is impaired in the mild stages of parkinsonism, even before the appearance of motor symptoms.