The genetic properties and the frequency of cancer family syndrome (CFS) were evaluated on the basis of 22 kindreds identified in Finland by investigating the family histories of young patients with colorectal carcinoma. These families had 196 members with malignancies: 120 colorectal (61%), 30 undefined intraabdominal (15%), 20 endometrial (10%), 7 gastric (4%), 5 biliary tract (3%) adenocarcinomas, and 14 (7%) other cancers. The present CFS patients represented 0.4% of all colorectal cancers diagnosed in Finland during 1961-1980, and the estimated minimum frequency of CFS was 4-4.7 X 10(-5) (1 in 25,100-21,400). An autosomal dominant pattern of genetic transmission for colorectal cancer was demonstrated through two to four generations, but the trait was also transferred by mothers with uterine carcinoma. The highest incidence of colorectal carcinoma occurred between 40 and 49 yr of age (15% per decade), and the cumulative risk in the descendants increased to 50% at 69 yr. The penetrance of CFS was 0.5-0.9. The analysis suggest that heredity is more significant in the etiology of colorectal cancer than has been previously believed. Identification of CFS seems to provide a good opportunity for screening the descendants of affected patients for colorectal cancer.