To examine a possible relationship of essential tremor to Parkinson's disease (PD), 130 patients with essential tremor were studied. Twenty-five patients had both essential tremor and PD. These 25 patients were matched for age and duration of PD symptoms with 25 patients who had idiopathic PD. Bradykinesia, postural instability, and gait difficulty were more severe in the patients with idiopathic PD. Degree of tremor, rigidity, and functional disability did not differ in the two groups. The prevalence of PD in the population with essential tremor was 24 times greater than expected. This study suggests that some patients with essential tremor have a genetically increased risk for PD.