The high frequency of cystic fibrosis (CF) mutations in males with absence of vas deferens supported the hypothesis of a primarily genital phenotype of CF disease. To consider the idea of an attenuated form of CF, we investigated 14 men with congenital bilateral aplasia of the vasa deferentia. All patients were consulting for infertility and none was known to have CF. The median age was 30.5 years (range, 20-38 yr). DNA analysis for 22 CF mutations showed at least 1 mutation in 10 patients (71%), whereas the CF carrier frequency is only 4% in the general population. Three compound heterozygotes were identified, all carriers of the R117H mutation. The sweat test was considered positive in 6 patients (43%), and a high frequency of radiologic evidence of sinus disease (8 patients) and of elevated antibodies to Pseudomonas (8 patients) was found. Only 2 patients were free of all these criteria for CF disease. This study strengthens the hypothesis that absence of vas deferens is an attenuated form of CF. We propose a combination of tests including DNA study, computerized tomographic scan of the paranasal sinuses, and testing of anti-Pseudomonas antibodies when the sweat test is inconclusive.