We analyzed the frequency of the G542X mutation in a sample of 76 Mexican cystic fibrosis patients and the genotype-phenotype correlation. The mutation was screened using PCR-mediated site-directed mutagenesis, and was present on 7.2% of the CF chromosomes. This frequency is significantly higher than the worldwide frequency according to the Cystic Fibrosis Genetic Analysis Consortium (3.4%, p < 0.01), and similar to that reported in Spain (8%), which is in accordance with the ethnic origin of the Mexican population. All patients carrying G542X on at least one allele had mild to moderate pulmonary disease. In patients with hepatobiliary involvement, the frequency of G542X chromosomes was higher than the frequency of the mutation in all the Mexican CF chromosomes.