Cystic fibrosis, a Mendelian recessive disorder, appears when both alleles carry one mutation. However, the existence of so-called complex genotypes, consisting of mutation and polymorphism at the same allele, are also described. We report here on two different alleles with the same mutation but a different genetic polymorphism, found in two unrelated patients from Croatia. One allele was characterized as G542X+F508C and the other as G542X+ P1290P. Both alleles were found in patients whose second allele was of the DF508 variety.