The aim of this study was to obtain an estimate of the frequency of the delta F508 mutation in the Portuguese population, and of the tightness of its association with specific haplotypes. Furthermore, the genotype/clinical phenotype relationship and the feasibility of prenatal diagnosis were also investigated. The analysis of 42 cystic fibrosis (CF) families revealed that (1) 52% of CF chromosomes carry the deletion of codon 508; (2) there seems to be a positive correlation between the occurrence of the delta F508 mutation and the severity of the disease; and (3) fully informative prenatal diagnosis can be offered in 76% of at-risk pregnancies by using both genomic and allele specific oligonucleotide probes.