Although colorectal cancer is a disease of the older population, these tumors are not infrequent before the age of 55. Through the data of a population-based registry, we proposed giving a description of the clinical features of three groups of patients in whom the disease occurred at a relatively early age of onset (group I: < 40 yr; group II: 41-50 yr; group III: 51-55 yr). There were only 14 patients under the age of 40 yr (1.1% of total registered patients, n = 1298 in the period 1984-1992). Group II and III represented 5.9% and 6.0%, respectively (n = 76 and 78), with minor fluctuations throughout the 9-yr period of registration. Inherited colorectal tumors [hereditary nonpolyposis colorectal cancer (HNPCC), adenomatosis coli, and suspected HNPCC] accounted for 38.4% of group I patients (5 of 14), 17.1% of group II, 10.2% of group III, and only 3.5% of individuals older than 55 (p, for trend, < 0.001). Thus, hereditary colorectal tumors were detected significantly more often in younger individuals. The majority of colorectal malignancies were localized in the left colon or rectum in all three groups, with a tendency (not significant) to a preferential localization in the right colon for tumors developed in group I (37% vs 18% and 14% in groups II and III, respectively). Pathological stage and main histological types did not differ among the three groups. Finally, life-table analysis did not show significant differences in 5-yr survival among the three groups; however, when considered together, early onset cases showed a more favorable prognosis than older individuals (log-rank 11.6; p < 0.001). In conclusion, colorectal cancer is diagnosed very rarely before the age of 40 yr, whereas about 12% of all cases belong to the age group 41 to 55 yr of age. Hereditary tumors were found more frequently in younger patients, with a well-defined inverse relationship between age of onset and frequency of genetically determined tumors. Finally, the clinical outcome was more favorable in the whole series of early onset cases than in older registered patients.