Cystic fibrosis (CF) is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. We report on a novel nonsense mutation that leads to exon skipping and the activation of a cryptic exon. Screening of genomic DNA from 700 German patients with CF uncovered four cases with the nonsense mutation E92X, a G-->T transversion that creates a termination codon and affects the first base of exon 4 of the CFTR gene. Lymphocyte RNA of two CF patients heterozygous for E92X was found to contain the wild type sequence and a differentially spliced isoform lacking exon 4. In RNA derived from nasal epithelial cells of E92X patients, a third fragment of longer size was observed. Sequencing revealed the presence of E92X and an additional 183-bp fragment, inserted between exons 3 and 4. The 183-bp sequence was mapped to intron 3 of the CFTR gene. It is flanked by acceptor and donor splice sites. We conclude that the 183-bp fragment in intron 3 is a cryptic CFTR exon that can be activated in epithelial cells by the presence of the E92X mutation. E92X abolishes correctly spliced CFTR mRNA and leads to severe cystic fibrosis.