Movement‐related cortical potentials in response to simple, self‐paced, brisk index finger abduction movements were recorded in patients with simple and complex writer's cramp and compared with those of age‐matched control subjects. Analysis of the movement‐related cortical potential waveforms showed that the Bereitschaftspotential, the peak of the negative slope, and the frontal peak of the motor potential did not differ in the two groups, except for the average amplitude of the early part of the negative‐slope peak, which was decreased in the patient group during the interval of 300 to 200 msec prior to electromyographic onset. This finding was restricted to the electrodes overlying the contralateral and midline central electrodes. Movement‐related cortical potentials from patients and control subjects could be equally accounted for by a four‐dipole source model with sources located in the contralateral and ipsilateral sensorimotor regions and the supplementary motor area. There was a trend for a reduction in the strength of the sensorimotor sources active during the premotor period in the patient group, but the difference did not reach a significant level for any individual source. No differences were found between the movement‐related cortical potentials elicited by movements of the affected and unaffected hand, or between those of patients with simple or complex hand cramps. This result suggests a deficiency of contralateral motor cortex activation just prior to the initiation of voluntary movements in patients with focal dystonia.