The review of the available literature seems to suggest that the frequency of HNPCC in Caucasian populations ranges between a minimum estimate of 1 percent to a maximum of almost 6 percent of all colorectal carcinomas. The differences can be partly explained by the dissimilarities in patient selection, thoroughness of the pedigree analysis and selection of inclusion criteria. The most remarkable difficulty in estimates based on the family history is the fact, that the syndrome cannot be verified until several family members have been affected by cancer. Genetic tests which probably will be developed after cloning of the two HNPCC-genes, would make possible the diagnosing of single cases" (i.e. first mutation) of HNPCC in the near future. This may alter the previous estimates. However, the practical identification and recognition of Lynch syndrome families will continue to be based on a careful evaluation of the family tree until genetic tests will be adapted for routine practice in colorectal cancer.