Transcranial magnetic stimulation was used to investigate the properties of the corticomotor pathway and to map the primary motor cortex projection to hand and forearm muscles during a sustained isometric contraction in a group of subjects with writer's cramp of varying,duration, Corticomotor threshold, motor evoked potential amplitude and latency, and silent-period duration were normal on both sides in all subjects. The maps of the corticomotor projection were displaced relative to normal in all subjects, and in some cases were distorted in shape, with extensions of the lateral borders and the emergence of almost discrete secondary motor areas. The degree of map distortion and displacement was greatest in subjects with long-standing writer's cramp >5 years), and was bilateral in some cases. Injection of botulinum toxin into affected muscles demonstrated that the alterations in map topography were not fixed, and could be temporarily reversed during the period when the clinical effects of the injection were greatest, with the maps returning to their original positions as the effects of the injection wore off. It is concluded from this study that there are slowly evolving reorganizational changes in the primary motor cortex in writer's cramp, and that these changes may be secondary to altered afferent inputs from both clinically affected and unaffected muscles.