The performance of a motor task not only requires subjects to plan, prepare, and initiate but also to monitor how a movement is performed. We used positron emission tomography to examine to what extent the human cerebellum is involved in controlling motor output or sensory input from movements in normal subjects. In the first study, we compared the active performance of a motor task (flexion and extension of the right elbow) to the passive execution of the same movements. Passive movements were driven by a motor with the arm fixed in a guide hinge. Active movements (compared to rest) elicited increases of rCBF mainly in the ipsilateral neocerebellar hemisphere and vermis of the posterior lobe. During passive movements, almost identical parts of the cerebellar hemispheres and vermis were activated (compared to the rest condition). The direct comparison of active and passive movement conditions revealed a small activation of the neocerebellar hemisphere of the posterior lobe and cerebellar nuclei ipsilateral to the movement. Approximately 90% of cerebellar neuronal activity was related to sensory input. In the second study, we compared the execution of a free selection joystick movement task to a condition in which subjects simply imagined the movements. The execution of movements (compared to rest) was associated with increases of rCBF in the ipsilateral neocerebellar hemisphere and vermis of the posterior lobe. During movement imagination, a small part of the ipsilateral cerebellar hemisphere and vermis of the posterior lobe was activated (compared to rest). The increase of rCBF during movement imagination accounted for only 20% of the signal seen during movement execution. Our results indicate that the neocerebellum may be much more concerned with sensory information processing than has been considered previously.