Diffuse panbronchiolitis (DPB) in Japan is a chronic bronchitis observed in nonsmoking adults, with severe obstruction and poor prognosis. DPB shares pathologic and clinical characteristics with mild adult cystic fibrosis (CF), except that CF is frequent in whites (Europeans and Americans of European descent) but not in Japanese. Recently, the cystic fibrosis transmembrane conductance regulator (CFTR) gene was identified, and a 3-base pair deletion (delta F508) was confirmed as a major mutation responsible for CF. We extracted genomic DNA from white blood cells of 17 DPB patients and from paraffin-embedded tissues of 4 DPB patients at autopsy. Two polymerase chain reaction (PCR) primers were made in exon 10 of the CFTR gene so that a three-base shorter segment of 78 base pairs was amplified from the CFTR gene with the delta F508 mutation; the DNA segment amplified from the normal gene contains an F508 area with 81 base pairs. Every DNA segment amplified from DPB patients showed a normal 81-base pair length, indicating no DNA sample contained the delta F508 mutation. These results based on delta F508 mutation analysis in the CF gene indicate that DPB may represent a disease different from CF.