Three hundred nineteen affected members of 94 Danish families with familial polyposis coli had been registered in the Danish Polyposis Register by the end of 1982. Of the 247 histologically verified cases, 168 were propositi and 79 were call-up patients. The frequency of colorectal cancer at the time of diagnosis of polyposis was 69 percent in propositi vs. 3 percent in call-up patients. The cumulative survival rate after ten years was 97 percent in call-up patients, as compared to 42 percent in propositi. The prognosis of polyposis patients has improved significantly since the establishment of the Danish Polyposis Register in 1971.