SUMMARYCentral motor reorganization was studied in 33 subjects with hemiplegic cerebral palsy. Corticospinal projections were investigated using focal magnetic stimulation of the motor cortex. Reflex pathways were examined with digital nerve stimulation. Cross-correlation analysis of multi-unit EMG was used to detect activity in branched common stem last order presynaptic inputs to motor neuron pools. The neurophysiological findings were related to the clinical outcome.In 21 of the subjects studied (64%), there was evidence for reorganization of central motor pathways. The clinical and neurophysiological findings revealed two different forms of reorganization. In both forms focal magnetic stimulation demonstrated novel ipsilateral motor pathways from the undamaged motor cortex to the hemiplegic hand. Ipsilateral projections were not demonstrated from the damaged motor cortex. Eleven subjects had intense mirror movements. In these subjects cross-correlation analysis and reflex testing suggested that corticospinal axons had branched abnormally and projected bilaterally to homologous motor neuron pools on both sides of the spinal cord. The remaining 10 subjects did not have intense mirror movements and in these subjects there was no evidence for last order branching of corticospinal axons.It was found that good function of the hemiplegic hand was associated with the presence of EMG responses in that hand following magnetic stimulation of the contralateral motor cortex. When EMG responses were absent, hand function was poor unless the subject had intense mirror movements.