Patients with Parkinson's disease (PD) often complain of blurred vision or even of distinctive visual disturbances like hallucinations and illusions. Recent studies have emphasized the potential influence of primary visual deficits of color and contrast discrimination. To study primary visual function, we studied color discrimination (CD) and contrast sensitivity (CS) during 'on' medication in PD patients and compared them to non-PD subjects. Twenty one PD patients were compared to 30 age-matched controls using CD tested by the D-15 Lanthony test (D15) and the Farnsworth-Munsell 100 Hue test (FM) and CS tested by the Pelli-Robson (PL) and the Vis-Tech tables (VT). We excluded subjects with a visual acuity </=0.6 Snellen fraction or known ophthalmological diseases. PD patients showed greater impairment on all visual tests than controls. This difference was significant for the FM test (P<0.001), the spatial frequencies 12 and 18 cpd (cycles per degree) of the VT test (P<0.05) and both the monocular and binocular PR tests (P<0.05). Most tests for CS and CD showed statistical independency. CS deficits, but not CD deficits, correlated with age in both patients and controls. This study documents major and independent impairment of both color and contrast discrimination in PD patients. Further studies should elucidate possible clinical implications and correlations, such as the frequency of falls or visual hallucinations.