Anecdotal reports suggest an increased frequency of certain cancers in patients with cystic fibrosis, the commonest genetic disorder of whites. One third of patients with cystic fibrosis now reach adulthood, when cancer is more frequent, implying that cancer rates in these patients will increase over time. We investigated the relation between cystic fibrosis and cancer in North American and European patients with cystic fibrosis. We performed a retrospective cohort study of the occurrence of cancer in 28,511 patients with cystic fibrosis from 1985 through 1992 in the United States and Canada. The number of cases observed was compared with the number expected, calculated from population-based data on the incidence of cancer. We also analyzed proportional incidence ratios to assess the association between specific cancers and cystic fibrosis in Europe. Thirty-seven cancers were observed in the North American cohort during 164,764 person-years of follow-up, as compared with an expected number of 45.6, yielding a ratio of observed to expected cancers of 0.8 (95 percent confidence interval, 0.6 to 1.1). Thirteen digestive tract tumors were observed, as compared with an expected number of two, for a ratio of observed to expected cancers of 6.5 (95 percent confidence interval, 3.5 to 11.1). In Europe, 11 of 39 cancers originated in the digestive tract, yielding a positive association between digestive tract tumors and cystic fibrosis (odds ratio, 6.4; 95 percent confidence interval, 2.9 to 14.0). Although the overall risk of cancer in patients with cystic fibrosis is similar to that of the general population, there is an increased risk of digestive tract cancers. Persistent or unexplained gastrointestinal symptoms in these patients should be carefully investigated.