The Stroop phenomenon was studied in patients with early and late onset Parkinson's disease (PD) and in normal controls to examine the effect of this disease on generating and controlling automatic mental processes. In the Stroop task subjects are presented with color or neutral words in various colors and asked to ignore the word and name its color as fast as possible. We examined the facilitory and interference effects of the irrelevant word upon naming the color by using computerized version of the Stroop test. Vocal reaction times of both early and late onset PD patients presented an augmented facilitory effect. In addition, error data of the late onset PD patients showed an enlarged interference effect. These effects are related to an impairment in the ability to control automatic-reflexive processes. The augmented facilitory effect is manifest early in the course of PD suggesting that the basal ganglia act to inhibit some automatic cognitive processes. The origin of the interference effect in late onset PD patients is less clear. It may reflect more severe basal ganglia dysfunction; or a decline in cortical inhibitory processes.