ALTHOUGH prefrontal leukotomy has been utilized as a psychosurgical therapeutic procedure since 1935, very little has been written about the organic changes which are observed in the brain following this procedure, and even less about observable roentgenographic changes to be seen with pneumoencephalography. It is the purpose of the present paper (1) to describe the pneumoencephalographic findings in 19 unselected cases before and after prefrontal lobotomy for variable periods of up to 20 months following the surgical procedure, and (2) to correlate the pneumoencephalographic findings with observations in the few autopsies performed at this institution and elsewhere. HISTORICAL REVIEW In June 1936, Egas Moniz published the results of "prefrontal leucotomy" on 20 patients on whom he had operated since Nov. 12, 1935. It was his theory, as stated by Freeman and Watts,1that "to cure these patients we must destroy the more or less fixed arrangements of cellular connections