German patients with cystic fibrosis (CF) were screened for molecular lesions in exon 13 of the cystic fibrosis transmembrane conductance regulator (CFTR) gene by single strand conformation polymorphism (SSCP) and chemical cleavage of mismatch analyses. Direct sequencing of four samples that displayed the same SSCP pattern and that were susceptible to cleavage of hetero-duplexes by osmium tetroxide revealed, in all cases, a deletion of a single T residue at nucleotide position 2143 within codon 671 of the CFTR gene. As a result, leucine codon 671 is changed into a termination codon. In total, the 2143delT mutation was confirmed in 6 out of 271 German non-delta F508 CF chromosomes by artificial restriction fragment length polymorphism analysis, indicating that this frameshift mutation accounts for about 2% of German non-delta F508 mutations. The 6 pancreas insufficient patients who are compound heterozygous for 2143-delT suffer from the typical features of pulmonary and gastrointestinal CF disease. The 2143delT mutation completes the panel of the more frequent CFTR mutations that reside on the "delta F508 haplotype" and that contribute to its overpresentation among German non-delta F508 alleles that are associated with severe forms of disease.