We evaluated sensorimotor processing in patients with writer's cramp using PET and H2(15)O blood flow scans. The study included six right-handed patients with unilateral writer's cramp and eight right-handed normals. Subjects had blood flow scans at rest and during vibration of either the "affected" or "unaffected" hand. Vibration produced a consistent peak response in primary sensorimotor area (PSA) and supplementary motor area (SMA), both contralateral to the vibrated hand. Both responses were significantly reduced approximately 25% in patients with writer's cramp (PSA, p = 0.002; SMA, p = 0.02) whether vibrating the affected or unaffected hand. This indicates that patients with unilateral writer's cramp have bilateral brain dysfunction. These data provide objective evidence of abnormal central sensorimotor processing in writer's cramp.