In recovered stroke patients, performance of motor tasks with the affected limb has been reported to activate cortical areas ipsilateral to the affected side. The better to determine the causal role these areas play in recovery of motor function, we assessed cerebral activation during motor activity longitudinally after hemiparesis due to cerebral infarction. A secondary goal was to ascertain the relation between mirror movements and activation ipsilateral to motor activity. Positron emission tomography with oxygen-15 water measured regional cerebral blood flow during wrist movement early and late in the course of recovery from hemiparesis. Surface electromyography recorded muscular activity, and computer-assisted video analysis quantified movement during the scans. Mirror movements, movements contralateral to the instructed movement of the hemiparetic arm, were often seen. Activation of motor areas in the hemisphere ipsilateral to the affected limb roughly correlated with presence of mirror movements. Other changes in cerebral activation were small, when the task was controlled for rate, but high-rate-specific recruitment of ipsilateral cortical areas occurred in one case. However, the common occurrence of mirror movements, particularly with effortful tasks, complicates interpretation of data regarding the role of the ipsilateral hemisphere in recovery.