Reorganization of descending motor pathways was explored in 33 subjects with hemiplegic cerebral palsy. Subjects were assessed neurologically and surface electromyographic recordings were taken from homologous muscles of both hands. Functional corticospinal projections were assessed using focal magnetic stimulation of the motor cortex. In control subjects this evokes EMG responses in the contralateral hand at short latency. Similar results were seen in 12 of the hemiplegic subjects following stimulation of the undamaged motor cortex. In the remaining 22 subjects novel corticospinal pathways were demonstrated arising from the undamaged cortex, where stimulation evoked short latency EMG responses in both hands. Cross-correlation analysis performed from EMGs recorded between the two hands revealed short duration central peaks in 11 of these subjects, all of whom had strong mirror movements of the hands. These findings suggested that two patterns of central reorganization may follow early unilateral cortical insult. Examination further indicated that hand function in hemiplegic subjects could be related to the neurophysiological results.