The learning ability of 12 patients with Parkinson's disease (PD) was studied using a verbal paired-associate learning task, and was compared with that of ten patients with Alzheimer's Disease (AD) and 12 controls (NC). Overall, the PD patients performed significantly better than the AD patients but significantly worse than the NC subjects. Their performance was not related to their overall level of cognitive functioning as measured by the Mattis' Dementia Rating Scale, but was unequally distributed suggesting that the PD population actually consisted of more than one subgroup. A low error group performed like controls, while a high error group had a learning impairment comparable to that of AD patients. It is concluded that PD patients may have three patterns of neuropsychologic performance: some are unimpaired, some have "focal" abnormalities, and some have a generalized impairment of cognitive function.