Possible predictors of reported lower cognitive functioning in irradiated children with acute lymphoblastic leukemia (ALL) were investigated. Thirty-four subjects, 5-14 years old, with ALL in continuous complete remission and without evidence of current or past central nervous system disease, were examined 9-110 months after diagnosis, using standard measures of intelligence and academic achievement. Subjects with a history of post-irradiation somnolence syndrome were significantly older at diagnosis than nonsomnolent subjects. Intelligence (IQ) was found to be unrelated to history of somnolence syndrome. IQ and achievement were unrelated to age at irradiation, irradiation-examination interval, and radiation dosages. The strongest predictor of IQ by far is parental social class. The importance of controlling for social class differences when searching for treatment effects on IQ and achievement is stressed.