Readiness potential is a slow negative shift of brain electrical potential preceding voluntary movements. The amplitude of the potential is maximum over the area of the contralateral motor cortex which represents contraction of muscles. But this is distributed in a rather diffuse fashion and can also be obtained in bilateral regions. In this study, the readiness potential was obtained by averaging the EEG synchronized with repetitive hand movements. The potentials were recorded in the central scalp region in two groups; Group 1 as control consisted of six healthy right-handed persons, and Group 2 of seventeen patients with occlusive cerebro-vascular disorders who had varying degrees of motor weakness and who had undergone superficial temporal artery-middle cerebral artery anastomosis. The potentials in group 2 were obtained by carrying out voluntary movements on the unaffected side, if not mentioned. The relationship between preoperative readiness potentials and surgical results was studied. 1. In the control group, hand movements elicited the potential (N) with the amplitude being 7.7±0.8 μV (mean+standard error of means) on the ipsilateral side to the movement and 8.9±1.0 μV on the opposite side. N on the opposite side was significantly higher than on the ipsilateral side (p<0.02 t-test). 2. In group 2, N was 3.2±0.7 μV on the side of the hand movements and 5.4±0.8 μV on the opposite side. These values were signigicantly lower than those in group 1 (p<0.02 t-test). 3. Four patients with normal amplitudes (more than 5 μV) on the scalp of the lesioned side showed good results from surgery. On the other hand, four patients with very low amplitudes (less than 2 μV) had poor results. One patient out of eight with low amplitude (between 2 μV and 5 μV) showed good results, five patients showed fair results, and the other two patients showed poor results. 4. Readiness potentials after surgery were recorded in 10 patients. N was 3.6±0.8 μV on the ipsilateral side to the movement and 6.0±1.4 μV on the opposite side. 5. Readiness potentials recorded by repetitive hand movements of the affected side were mostly very low in voltage (less than 2 μV ). These results suggest that readiness potentials could be a useful criteria for STA-MCA anastomosis.