Visual spatial contrast thresholds and suprathreshold contrast matches were measured before and after adaptation to high-contrast sinewave gratings in patients with Parkinson's disease (n = 27), patients with multiple system atrophy (n = 6) and a group of age-matched control patients without CNS disease (n = 27). Contrast thresholds were higher in the Parkinson's disease patients than in either the multiple system atrophy patients or control patients. The effect of contrast adaptation on both contrast thresholds and matches was approximately equal in the three groups. This suggests that contrast adaptation is not affected by these CNS disorders and is consistent with the hypothesis that the loss in contrast sensitivity in Parkinson's disease is mediated by retinal effects. The results are discussed in terms of the underlying pathology of the visual deficits in Parkinson's disease and the possible diagnostic implications for differentiating Parkinson's disease and multiple system atrophy.