Adenocarcinomas of the colon and rectum in persons under 40 years old. A population-based study. Data collected by nine population-based tumor registries participating in the Surveillance, Epidemiology, and End Results Program of the National Cancer Institute were analyzed to characterize the epidemiology of noncarcinoid adenocarcinomas of the colon and rectum in young adults. Tumors diagnosed in persons under 40 years old between 1973 and 1984 (n = 1736) were compared with those in persons 40 years and older (n = 106,760). This first large U.S. population-based study of colorectal adenocarcinomas in the young shows a higher incidence in blacks than whites and later detection in black males. It also shows a higher proportion of tumors of mucinous and signet ring histological type than in older age groups. Among the younger group, the average annual age-adjusted incidence rate was 34% higher in black males than in white males (12.6 vs. 9.4 per million persons) and 46% higher in black females than in white females (13.0 vs 8.9 per million persons). The proportion of tumors that were right-sided varied by age: 0-29 years, 30%; 30-39 years, 26%; 40-49 years, 22%; 50-59 years, 21%; 60-69 years, 24%; 70-79 years, 30%; and 80+ years, 35%. Males under age 40 were less likely to present with localized disease (whites, 27%; blacks, 21%) than were those aged 40 and older (whites, 39%; blacks, 36%). The proportion of tumors classified as mucinous decreased with age, from 28% among those aged 0-19 years to 5% among those 40 years and older. A similar trend was observed for signet ring tumors. Although this latter type accounted for 10% of large-bowel tumors among subjects aged 0-19 years, this proportion decreased with age to 0.2% in those 40 years and older. 