School attendance and school achievement were the parameters studied to assess the pediatric cancer patient's ability to learn and keep pace with their peers. Effects of CNS prophylaxis, as either intrathecal methotrexate (IT) alone or intrathecal methotrexate given in addition to cranial radiation (CRT), were studied in two groups. A third group of cancer patients who received no CNS prophylaxis, and two comparison groups, siblings and a matched sample of children, also participated in the study. Impairment in central nervous system function was measured by means of psychological testing, neurological examination, and computer-assisted tomography. Patients who received central nervous system prophylactic treatments at an early age had poorer performance on verbal IQ scores, with comprehension and arithmetic subscores being most affected. Patients who received both cranial radiotherapy plus intrathecal methotrexate showed a decrease in six out of seven categories of instruction when grades from the year prior to diagnosis were compared to those obtained 3 years after diagnosis. The combined groups of patients with leukemia had a lower grade point average and poorer school attendance than did the comparison groups.