The incidence of hereditary nonpolyposis colorectal cancer (HNPCC) is not precisely known. Common estimates are 4 to 6% of all colorectal cancers (CRCs), but lower figures have been published. In an attempt to obtain an independent new estimate of this proportion and to identify more HNPCC families, we designed a new method. Based on the fact that age at diagnosis is considerably lower in hereditary than in sporadic CRC, a cohort of 227 CRC patients aged 44 years or younger diagnosed during the years 1985-1989 and reported to the Finnish Cancer Registry was chosen as probands. Pedigrees of the probands were constructed by combining information from the Finnish Population Register Center and Finnish Cancer Registry using only automatically processed data. Sixteen pedigrees were extended further by the help of parish registers. Six of these turned out to be new possible HNPCC families but none met the stringent criteria of the cancer syndrome. Our results can be interpreted to suggest that the proportion of HNPCC of all CRCs is between 0.5 and 0.9%, depending on the diagnostic criteria used. Modifications of our method should be useful in other research projects dealing with diseases in which a subset is due to genetically or environmentally determined susceptibility.