Patients with Parkinson's disease have difficulty in performing two different tasks simultaneously. The present study tested whether this deficit was specific to the disease or was found in other patient groups. An identical pattern of performance was shown by a group of patients with cerebellar disease and, to a lesser extent, by a group of patients with Huntington's disease. Further research should focus on clarifying the nature of the deficits and the reasons for the similar performances in the various patient groups.