In order to study the importance of the psychic effect in the therapeutic efficacy of frontal lobotomy, as well as to compare the results of unilateral and bilateral operation, the authors studied thirty-three chronic psychotic patients. In these the operation was performed on one or both sides by chance selection and the extent of operation was kept unknown to the staff who were responsible for the postoperative care and studies. Six months after operation, beneficial results were obtained in sixty-eight and eight tenths percent of those who had undergone bilateral surgical intervention, as compared to seventeen and six tenths percent of the unilaterally-operated patients. In the latter group, a second contralateral operation was then performed on fourteen of the seventeen cases with a subsequent improvement six months later in eleven patients, or seventy-eight and six tenths per cent. The authors conclude that the psychic influence is not an important factor in the therapeutic results and that bilateral operation is the operation of choice.