The computed tomograms performed on a series of 41 patients during the course of prolonged treatment for acute leukaemia were reviewed. Thirty-one of these cases were referred for some neurological disturbance, the remainder for routine assessment. Abnormalities were present in 59%. In two cases there were leukaemic deposits, four were due to infection and four to necrotising leucoencephalopathy. Diffuse cerebral swelling occurred in three patients, and ventricular dilatation in 15, associated with enlarged cortical sulci in 13. The relationship of these findings to drug therapy is discussed.