Regional cerebral blood flow (rCBF) was measured with H2(15)O positron emission tomography in 5 patients with acquired hemidystonia (AHD) due to structural lesions of the basal ganglia or posterior thalamus contralateral to the dystonic limb. Patients were scanned at rest and when performing paced joystick movements in freely chosen directions with the dystonic and then the unaffected arm. Findings were compared with those of 5 age-matched controls performing joystick movements with the right arm. At rest, there was decreased activity in ventroanterior thalamus, posterior thalamus, angular gyrus ipsilateral to the lesion, and bilateral frontoorbital cortex. At a similar level of significance, increased resting activity was found in lentiform nucleus, hippocampus, and anterior insula contralateral to the lesion. Using the affected arm, AHD cases showed significant overactivity of contralateral prefrontal, lateral premotor cortex, rostral supplementary motor area, anterior cingulate area 32, bilateral sensorimotor cortex (SMC) and insula, mesial parietal cortex, and ipsilateral cerebellum. There was similar frontal overactivity when the unaffected arm performed the joystick movements, though SMC and insula overactivity was contralateral rather than bilateral. The associated frontal overactivity on movement is consistent with acquired dystonia being a syndrome of thalamofrontal disinhibition due to structural disruption of basal ganglia inhibitory control. Our findings also suggest that cortical activation during movement of the unaffected limb is abnormal in acquired hemidystonia.