Trends in the incidence of childhood cancer in Connecticut are reported and analyzed for the period 1935-1979 by 5-year age groups (0-4,5-9,lO-14,15-I9 years), using a log linear model method. A threefold increase (P < .001) in the incidence of ALL in males 0-4 years of age was observed, with significant increases of smaller magnitude seen in males aged 5-9 and 15-19 and females aged 0-4 and 5-9. The incidence of central nervous system cancers also increased in several age groups for both sexes with the largest increase seen in males 0-4 years old. Significant increases in incidence of large magnitude were also observed for Hodgkin's disease, in males aged 15-19 years and females aged 10-19 years, for neuroblastoma in both sexes at ages 0-4 years, and for testis and ovarian cancer at ages 15-19 years. This study of trends in incidence of childhood cancers by 5-year age groups has revealed significant changes, which would not have been as apparent if broader age groups had been used. These results provide relevant data for investigating the etiology of cancer during infancy, childhood, and adolescence. Trends in Connecticut are compared with findings from other registries in the United States and other countries.