To assess the relationship between the genotype and phenotype of adult CF patients we have selected from a group of 512 CF patients attending centres in France, all these of greater than 35 years. We have analysed the entire coding sequence of their CFTR genes. The complete genotype was determined in 7 of the 8 patients and clinical data regarding pancreatic, respiratory and reproductive function were carefully evaluated. All these patients are compound heterozygote, seven carrying the delta F508 and one the G542X on one allele. The other allele carried is: (i) a missense mutation located in exons coding for transmembrane region in five patients [R334W (1); I336K (2); R117H (1); H1054D (1)]; (ii) a splice mutation in two patients [2789 + 5G-->A], (iii) an uncharacterised mutations in one patient. These results strongly suggest less severe CF phenotype to be associated with these mutations and strengthen the hypothesis that less severe phenotype are genetically determined.