The alien hand sign (AHS) is often encountered in patients with corticobasal ganglionic degeneration (CBGD), revealing a unilateral dysfunction of the motor system of unknown pathophysiology. We examined the possibility of an abnormal cortical representation of hand muscles in 10 patients with probable CBGD and a prominent AHS. Cortical maps were obtained from the responses to magnetic stimuli applied with a figure of eight coil at an intensity of 110% above motor threshold. For comparison, the same study was carried out in 10 normal volunteers, eight patients with Parkinson's disease and eight patients with Alzheimer's disease. AHS patients had a larger extension of the cortical map to stimulation of the hemisphere contralateral to the AHS in comparison with the ipsilateral hemisphere. Furthermore, in six patients, focal stimulation of the hemisphere ipsilateral to the AHS gave rise to ipsilateral responses, delayed by a mean of 7.7 +/- 2.2 ms with respect to those recorded in the same muscle to contralateral stimulation. None of the other patients or control subjects had ipsilateral responses. Our results indicate an enhanced excitability, or reduced inhibition, of the motor area of the hemisphere contralateral to the AHS. The delay of the ipsilateral responses is compatible with a disinhibited transcallosal input.