The authors screened 374 patients clinically diagnosed be affected by cystic fibrosis. Mutations delta F508, G542X, G551D, R553X, N1303K were analysed to obtain genetic diagnosis. The large number of patients involved in this study allowed for authors to present precise data of the frequencies of these mutations in Hungary. The frequency of mutation delta F508 is found to be significantly less then the numbers reported in other studies. This is due to sampling bias occurring at little sample sizes. Mutational analysis has been used as a tool of prenatal diagnosis.