A group of children with acute lymphocytic leukemia was studied to investigate if a reduction in daily dose fraction of cranial radiation would reduce the incidence of somnolence syndrome. Thirty-one evaluable patients received 100 rad X 18 cranial radiation therapy. Sixty-six similar evaluable patients were given 180 rad X 10. Both groups received the same chemotherapy including intrathecal methotrexate. Clinically detectable somnolence appeared in 58% of ech group without significant differences in the overall frequency or severity of somnolence (p greater than 0.5). This study failed to substantiate a radiation dose fraction size dependence for somnolence syndrome in children with acute lymphocytic leukemia.