We used positron emission tomography (PET) with 15O-labelled water to record patterns of cerebral activation in six patients with Parkinson's disease (PD), studied when clinically “off” and after turning “on” as a result of dopaminergic stimulation. They were asked to imagine a finger opposition movement performed with their right hand, externally paced at a rate of 1 Hz. Trials alternating between motor imagery and rest were measured. A pilot study of three age-matched controls was also performed. We chose the task as a robust method of activating the supplementary motor area (SMA), defects of which have been reported in PD. The PD patients showed normal degrees of activation of the SMA (proper) when both “off” and “on.” Significant activation with imagining movement also occurred in the ipsilateral inferior parietal cortex (both “off” and when “on”) and ipsilateral premotor cortex (when “off” only). The patients showed significantly greater activation of the rostral anterior cingulate and significantly less activation of the left lingual gyrus and precuneus when performing the task “on” compared with their performance when “off.” PD patients when imagining movement and “off” showed less activation of several sites including the right dorsolateral prefrontal cortex (DLPFC) when compared to the controls performing the same task. No significant differences from controls were present when the patients imagined when “on.” Our results are consistent with other studies showing deficits of pre-SMA function in PD with preserved function of the SMA proper. In addition to the areas of reduced activation (anterior cingulate, DLPFC), there were also sites of activation (ipsilateral premotor and inferior parietal cortex) previously reported as locations of compensatory overactivity for PD patients performing similar tasks. Both failure of activation and compensatory changes are likely to contribute to the motor deficit in PD. © 2001 Movement Disorder Society.