Thirty-two asymptomatic patients with acute lymphocytic leukemia, who had received prophylactic cranial radiation (2400 rads) and either intrathecal methotrexate or cytosine arabinoside were studied by computed tomography of the brain 19 to 67 months after initiation of prophylaxis. Seventeen of 32 (53 per cent) had one or more abnormal findings. Dilatation of the ventricles (eight patients) and widening of the subarachnoid spaces (nine patients) were equally distributed among patients in both intrathecal-chemotherapy groups. Areas of decreased attenuation coefficient (hypodense, abnormally radiolucent regions) (four patients) and intracerebral calcification (one patient)--lesions previously described in methotrexate leukoencephalopathy--were found only in those who had received intrathecal methotrexate. Mild central-nervous-system dysfunction was detected in seven patients but did not correlate with the presence of tomographic abnormalities. Nevertheless, these tomographic findings may represent preclinical lesions. The unexpectedly high prevalence of such abnormalities contrasts with the essentially normal tomographic findings in a control group with acute lymphocytic leukemia who received no central-nervous-system prophylaxis. These results suggest that alternative approaches to such prophylaxis be considered.