The study assessed cognitive decline in non-demented, non-depressed patients with well defined Parkinson's disease and determined the predictive value for cognitive decline of different motor symptoms. Motor disability was measured with the Unified Parkinson's disease rating scale, impairment in activities of daily living, levodopa test, and long term clinical follow up. Neuropsychological evaluations included modified mini mental state, fluency, Wechsler logical memory, Wisconsin card sorting test, and the Montgomery and Asberg depression rating scale. Fifty three patients fulfilling clinical criteria for idiopathic Parkinson's disease were studied. Cognitive performance on initial testing was significantly correlated with education and disease duration but not with age at disease onset. Cognitive performance on retesting after three years of follow up was significantly reduced. This reduction was significantly greater in the late onset group, in patients with isolated dystonic dyskinesiae, and in patients with a lower percentage of motor improvement on levodopa. Cognitive decline in idiopathic Parkinson's disease may depend on both the prevalence of non-dopaminergic lesions and the topography of dopaminergic denervation. Predictive factors for cognitive decline, especially in executive tasks, relate more to non-dopaminergic than to dopaminergic lesions.