Collecting of patients diagnosed to have familial adenomatous polyposis coli (FPC) in Finland and a centralized systematic screening of their family members were undertaken by the Depts. of Surgery of the five Finnish university hospitals. In all, 160 FPC patients belonging to 50 families could be identified, and they had 178 descendants at 50% risk. The frequency of FPC was 5.63 X 10(-5) (1:18,000), which may represent an underestimate. It gives rise, however, to four new FPC patients in Finland every year. The distribution of the origin of the FPC families was even over central Finland, but small aggregates, probably branches of the same families, were found. Of the 50 families, 28 (56%) had at least 2 affected members, whereas 22 had only 1. The gene penetrance was estimated to be 80-89% at minimum. Children of solitary FPC patients must be considered to have the same risk of inheriting the gene as do those of established familial patients. The necessity of a permanent national centralized organization for the screening and management of FPC is emphasized.