Voluntary finger-nose movements of the arm were analysed in six patients undergoing stereotaxic nucleus ventralis intermedius thalamotomy for relief of severe Parkinsonian tremor. In all cases thalamotomy acutely abolished tremor in the contralateral arm. In the early postoperative phase, ataxia of the arm contralateral to the operated side was also seen. Ataxia was transient, lasting between 7 and 21 days postoperatively. This observation suggests that a lesion of the Vim nucleus interrupts cerebellar input to the thalamus, and supports the concept that abnormal cerebellar activity is an important contributor to the generation of tremor in Parkinson's disease.