Patients with Parkinson's disease and controls traced patterns of increasing complexity presented on a vertical transparent screen. Some patterns were presented with missing segments which subjects were required to fill in. Digitising equipment quantitated movements for computer analysis. Subjects also completed a brief test of general intellectual function, a construction test and an assessment for depression, and the severity of patients' Parkinsonian signs and symptoms was rated. Patients performed more poorly than controls on complete patterns, and their errors increased more sharply than controls on patterns with missing segments. Patients' errors, but not those of controls, in filling in missing segments were uniquely related to performance on construction tasks. Other aspects of patients' tracing performance correlated with the severity of their Parkinsonian symptoms. These findings suggest that there is an underlying perceptual motor deficit in Parkinson's disease that affects performance on both tracing and construction tasks.