Death certificates of 2642 U.S. children who died of non-Hodgkin's lymphoma, 1960–67, and hospital charts of 900 children with these neoplasms were studied. Lymphosarcoma showed the highest morbidity throughout childhood, with small age peaks at 3–5 years for both sexes. In contrast, reticulum cell sarcoma increased gradually with age. For all cell types combined, non-Hodgkin's lymphoma was 2½ times more common among males than females under age 15. The highest sex ratio characterized lymphosarcoma of the gastrointestinal tract (6:1 male excess). As a feature shared by Hodgkin's disease in childhood, male preponderance seems to reflect a sexrelated susceptibility common to all forms of solid lymphoreticular neoplasms. Transitions to adult age and sex patterns occurred in late childhood. There was no geographic or temporal variation in mortality to suggest major environmental influences. Mortality time trends between 1950–69 revealed a significant decline in lymphosarcoma and increase for reticulum cell sarcoma and other cell types. In the hospital series, a few children had altered immune states, celiac malabsorption, or radiation exposure. Sibs of children with non-Hodgkin's lymphoma showed an increased risk for lymphoma, leukemia, and brain neoplasms. The frequency of leukemic transformation in non-Hodgkin's lymphoma was greatest in early childhood, affecting 34% of children under 5 years. Epidemiologic comparison between childhood lymphoma and leukemia showed similarities as well as important differences.