Regional cerebral blood flow responses to vibrotactile stimulation were studied in 11 patients with predominantly unilateral idiopathic focal dystonia and 18 normal subjects using PET and H2(15)O. Stimulation produced a consistently localized and robust peak response in primary sensorimotor cortex contralateral to hand vibration in normal subjects (averaged hemisphere response 10.97 ml/(100 g.min) +/- 2.53). The sensorimotor response in dystonic patients was also consistently localized to the same area, but significantly reduced in magnitude whether vibrating the affected (8.35 ml/(100 g.min) +/- 2.29) or unaffected hand (8.40 ml/(100 g.min) +/- 2.15). Furthermore, vibration induced a dystonic cramp in the stimulated arm/hand in 6 patients, but not in any normal subjects. To determine whether the cocontraction of agonist and antagonist muscles could, in itself, attenuate the regional blood flow response, 10 normal subjects were studied with vibration during voluntary cocontraction of appropriate hand and forearm muscles, as well as with vibration alone. Vibration with voluntary cocontraction (mean = 12.57 ml/(100 g.min) +/- 2.13) produced a significantly greater response than vibration alone (mean = 10.30 ml/(100 g.min) +/- 2.20, P less than 0.00008). This abnormal sensorimotor response may have important implications for understanding the pathophysiology of idiopathic dystonia.