The frequency of hereditary colorectal carcinoma was evaluated in a study group consisting of all colorectal carcinoma patients (n = 468) diagnosed in one Finnish county (0.25 million inhabitants) during the period 1970-1979. The cancer family syndrome type of hereditary nonpolyposis colorectal carcinoma emerged as the most common verifiable risk factor for colon cancer involving 3.8%-5.5% of all colorectal carcinoma patients in this study. The frequencies of familial adenomatosis and ulcerative colitis were 0.2% and 0.6%, respectively. As the diagnostic method of this study was based on the family history of the patients, only those families with inherited cancer cases in two or more generations could be identified. The cancer family syndrome cannot be diagnosed on the basis of a single patient, and so the observed frequency of 4%-6% for the syndrome may still represent an underestimate. The patients with cancer family syndrome were young, accounting for 29%-39% of the patients under 50 yr of age, and their tumors were located predominantly (65%) in the right hemicolon. The high frequency of hereditary cases among colorectal carcinoma patients indicates the importance of studying the family history of every new patient.