Leukemias and lymphomas account for nearly half of all childhood cancers. Although there have been major advances in the treatment of these diseases, what causes them remains largely unknown. There is strong evidence to suggest that leukemia originates in utero, and early life factors may play a role in its etiology. A series of reports illustrate a convincing link between the rate of intrauterine growth and the risk of childhood leukemia. Some studies suggest that this risk relationship also extends to non-Hodgkin lymphoma in children, although, overall, the association with childhood lymphoma is less clear. This review discusses the intricacies of these risk relationships and explores potential explanations of how the rate of fetal growth may influence cancer risk.