The neurophysiological aspects and operative technique of stereotactic limbic leucotomy have been described in a previous paper (Kelly, Richardson and Mitchell-Heggs, 1973). The present investigation is a prospective study designed to assess the results of such surgery in a group of 40 severely ill psychiatric patients, who had failed to respond satisfactorily to every other type of treatment. The results have been assessed clinically, psychologically and physiologically, in a very detailed way, at six weeks; a similar follow-up at one year is in progress. A comparison is made between the results of the present series and those of a previous: study (Kelly et al., 1972), in which more extensive leucotomy operations were carried out, and similar means of assessment were employed.