Patients with cystic fibrosis (CF) are at increased risk of some cancers. Little is known about the cancer risks among carriers heterozygous for the CF mutation and it is hypothesized this may be associated with reduced cancer risk. Using Swedish general population-based registers, we identified 884 patients with CF from 1968 to 2003 and 3,033 of their first-degree relatives The subjects were followed from birth of index persons or 1958, whichever came later, until death, emigration or 2003, whichever came first. Cancer risks were compared with the general Swedish population using standardized incidence ratios (SIR) with 95% confidence intervals (CI). Patients, followed for an average of 21 years, were at a higher overall risk of cancer. Some 26 cancer diagnoses, after excluding multiple diagnoses of nonmelanoma skin cancer in one man, produced an overall SIR of 3.2 (95% CI 2.1-4.6). We found statistically significantly increased risks for kidney, thyroid, endocrine, lymphoma and nonmelanoma skin cancer. There was no modification of cancer risk among parents and siblings, with an average of 21 years of follow-up. This study did not identify a heterozygote advantage for CF gene mutations in relation to cancer risk.