Twenty-three children with acute lymphocytic leukemia (ALL) were examined with cranial CT at least twice with a minimal interval of 10 months. The first CT was performed at the time of diagnosis in 11 children and during therapy in 12; all but two were normal on the first CT examination. These two had slight enlargement of the ventricular system and subarachnoid space at the time of diagnosis. These findings were unchanged on the second CT examinations. Seven patients, all in remission from leukemia of the central nervous system manifested abnormal findings on later CTs. Low density areas in the periventricular white matter were seen in the brains of three, with increasing subcortical calcification in one of these cases. Five children had slight enlargement of the ventricular system and subarachnoid space, especially of the basal and Sylvian cisterns. Later CT examinations in five, plus brain autopsy in two cases, revealed unchanged or progressive conditions. The CT findings have been related to the treatment and some characteristics of the disease. The frequency of CT abnormalities was higher in patients who had received therapeutic irradiation and intraventricular methotrexate treatment. The possible reasons for the CT abnormalities are discussed.