A report is presented on a series of 600 cases of prefrontal lobotomy done by a modification of the Lyerly technique. This series has been operated by the same surgeon and followed throughout by the same psychiatrists. The authors feel that the operation is therapeutically active, and is useful and practicable. The psychiatric response has so far been found to be stable and progressive. No late deteriorative or other late unfavorable psychic effects have been noted. About 1% of the patients are considered psychiatrically worse after the operation. "Personality changes" have not been a practical problem and complaints that could be classed in this category have been infrequent, of minor significance, and have tended to disappear in the course of the first year or two postoperatively. Schizophrenic defect states of the usual pattern and distribution have been noted. Spontaneous postlobotomy convulsions have not been found to exert a favorable clinical effect, although lobotomy patients show the usual symptomatic response to convulsions induced by electric shock. Spontaneous convulsions tend to occur in groups and seem to be more frequent in catatonic patients than in others(1).Nonschizophrenic cases represented approximately 10% of the operated series of 600 and 12% of the releases of the entire series. Release rates for patients of 3 years or less of hospital residence were definitely better than those of longer hospital residence, and it was also found that the quality of remission was less satisfactory in older cases. A very important effect of the operation, behavior improvement, developed in even the most chronic cases although in smaller proportion than in less chronic ones. Approximately two-thirds of all operated cases remaining in the hospital showed material improvement in such behavior disorder as aggressiveness, destructiveness, wetting, soiling, homosexual behavior on a psychotic level, refusal of food, etc.After a period of postoperative observation that now ranges from 1 to 4 years, it appears that approximately one-third of all operated cases have been able to return to their homes and to remain there. This proportion is very much higher than what one could expect from a group of unoperated cases of similar clinical picture, similar duration of hospital residence, and similar diagnosis.Prefrontal lobotomy was carried out for clear-cut indications in one case of moderately advanced postencephalitic parkinsonism, one general paretic, one case of muscular dystrophy, one mild case of congenital cerebral palsy.The psychiatric response of these cases was not notably different from that seen in "functional cases" with regard to psychotic manifestations, and in no case was there any intensification of the primary organic condition. No beneficial effects were noted after lobotomy in 2 cases of primary psychopathic behavior.