The bronchial epithelium in stepwise transverse sections was examined histologically in 66 male autopsy cases, composed of the groups of 19 mustard gas (MG) ex-workers with lung cancer, 17 MG ex-workers with non-lung cancer, 10 non-MG lung cancer cases, and 20 non-MG non-lung cancer cases. Foci of moderate or severe atypical cellular lesion or dysplasia, or of carcinoma in situ (CIS) in total slides of each group, were counted as 146 in 3,485, 72 in 2,226, 70 in 3,797, and 18 in 4,611, respectively. The relative frequency of moderate or severe dysplasia and CIS in MG exposed non-lung cancer cases resembled that found in lung cancer cases of both MG and non-MG exposed. Seven CIS lesions were detected from among all MG-exposed cases and one CIS was found in a non-MG lung cancer case. Six out of eight CIS examples were adjoined by dysplasia. A multi-variate analysis revealed a significant correlation between the incidence of atypical lesions and MG exposure, though the incidence of atypical lesions was also influenced significantly by age, smoking, and chronic bronchitis. The incidence of atypical lesions was significantly higher in cases of squamous cell lung cancer than those of other histological types, particularly small cell cancer.