We studied contrast-sensitivity function in 39 patients with Parkinson's disease. Sixty-four percent of the patients showed contrast-sensitivity loss in one or both eyes. The abnormality was not related to the first symptom or the severity of disease. Sensitivity loss at intermediate frequencies (notch loss) in 30% of the "affected" eyes suggested a cortical component. These findings support the belief that there is a widespread neurotransmitter deficiency in Parkinson's disease.