The cystic fibrosis (CF) gene was recently cloned and a three basepair deletion removing phenylalanine 508 from the coding region was identified as the mutation on the majority of cystic fibrosis chromosomes. The frequency of this mutation varies among different geographic locations. In this study we analysed 25 Turkish families and have found the relative frequency of the delta F508 (DF508) mutation to be 20.37%. This is similar to that reported in other Middle Eastern countries and contrasts with the considerably higher frequencies encountered in Northern Europe and North America. Molecular analysis of the two nucleotide binding folds (NBFs) of the cystic fibrosis transmembrane conductance regulator (CFTR) gene with direct sequencing, has also shown that the other defined mutations in these regions of the gene do not occur in the CF patients from the Turkish population. One patient presenting with severe clinical features was determined to be a compound heterozygote for the mutations delta F508/1677delTA.