Writer's cramp, or focal hand dystonia, is characterized by involuntary coactivation of antagonist or unnecessary muscles while writing or performing other tasks. Although the mechanism underlying this muscle overactivation is unknown, recent studies of changes in cerebral blood flow during writing have demonstrated a reduction in the activation of the primary motor cortex (MC) and hyperactivity of parts of the frontal non-primary motor areas. Therefore, any measures that decrease the activities of non-primary motor areas such as the premotor cortex (PMC) and the supplementary motor area (SMA) might improve dystonic symptoms. To explore this possibility, we studied nine patients with writer's cramp and seven age-matched control subjects, using subthreshold low-frequency (0.2 Hz) repetitive transcranial magnetic stimulation (rTMS), which exerts an inhibitory action on the cortex. Previous studies have demonstrated shortened cortical silent periods in dystonia, suggesting deficient cortical inhibition in the MC. We compared the silent periods and computer-assisted ratings of handwriting before and after rTMS applied to the MC, SMA or PMC. We also used the sham coil for control runs. Stimulation of the PMC but not the MC significantly improved the rating of handwriting (mean tracking error from the target, P = 0.004; pen pressure, P = 0.01) and prolonged the silent period (P = 0.02) in the patient group. rTMS over the other sites or using a sham coil in the patient group or trials in the control group revealed no physiological or clinical changes. This increased susceptibility of the PMC in dystonia suggests that the lack of inhibition in the MC is secondary to the hyperactivity of PMC neurons. Inhibition of the PMC using rTMS could provide a therapeutic measure of writer's cramp.