In an attempt to identify possible adverse effects of CNS prophylaxis (cranial radiation and intrathecal chemotherapy), we examined hypothalamic-pituitary function in 23 patients with acute lymphocytic leukemia (ALL). Of 18 patients who had received both cranial radiation and intrathecal chemotherapy, nine had abnormally low growth hormone responses to insulin-induced hypoglycemia (less than 7.0 ng/ml). Seven of the nine patients with abnormally low growth hormone responses also manifested ventricular dilatation on computed tomography (CT) brain scans, whereas only one of the nine patients with normal growth hormone responses demonstrated this CT scan finding (P = 0.015). The remaining patients, who had not received cranial radiation, had normal growth hormone responses and normal CT scans. There is significant correlation between ventricular dilatation on CT and abnormally low peak growth hormone responses following CNS prophylaxis in ALL.