Children receiving a bone marrow transplant (BMT) are at risk for neuropsychological late effects because of potentially neurotoxic chemotherapy and total body irradiation. The goal of this study was to prospectively and longitudinally assess the intellectual and adaptive functioning of children receiving a BMT. This study examined 67 children whose development was evaluated at baseline prior to BMT and at 1 year follow-up. Mean age at BMT was 45 months. Repeated-measures ANOVA indicated a significant decline in IQ between baseline and the 1 year follow-up evaluation. Multivariate and exploratory univariate analyses examined the potential influence of diagnosis, treatment regimen, cranial radiation dose, age at time of transplant, and sex of child but none of these independent variables predicted outcome. Twenty-six children (mean age at BMT of 28.4 months) were also given developmental evaluations 3 years post-BMT. Although IQ at the 1 year follow-up was significantly lower than baseline, no further changes were evident at the 3 year follow-up evaluation. Scores on the Vineland Adaptive Behavior Scales also dropped significantly between baseline and the 1 year follow-up, but did not change between the 1 year and 3 year evaluations.