Forty children newly diagnosed with acute lymphoblastic leukemia (ALL) were examined by computed tomography (CT) of the central nervous system (CNS) on hospital admission before any medication was started. The results of the CT scans were defined as normal, borderline (slight or moderate dilatation of the ventricular system and/or basal cisterns and/or convolutional sulci), or pathologic (severe cerebral atrophy). The mean age of the patients was 5.8 years (range 1.7-15 years). Sixteen of the 40 patients (40%) had CT scan abnormalities with 14 patients having borderline scans and two patients pathologic scans. No child presented with neurologic symptoms or CNS leukemia. These data suggest that CT abnormalities of the brain are common in children with ALL at diagnosis and may represent clinically unsuspected lesions secondary to leukemia.