We evaluated, by deterministic computer simulation, some effects of a screening programme for carriers of cystic fibrosis mutations. Two different selective regimes (heterozygote advantage and directional selection against recessive homozygotes) and three kinds of response to the screening were simulated. The curves describing the expected decline in the frequency of CF homozygotes allow one to predict some benefits of a screening campaign. In addition, it is shown that a strategy aimed at testing couples, rather than individuals, may become less expensive after only two generations of screening. The main source of uncertainty for a screening programme remains the selection mechanism, namely the existence of some sort of biological advantage for heterozy,gous carriers of CF mutations.