This study was done to investigate which histologic type of lung cancer is prevalent among male Japanese copper smelter workers. A panel of eight pathologists was asked to diagnose uniformly prepared materials for 19 occupational series, 87 nonoccupational bronchogenic carcinomas, and 14 benign lesions. The consensus diagnosis was used as reference. The reference diagnoses and the originals without materials employed for verification were designated as finals. Squamous cell carcinoma was the most frequent cell type in the occupational group, which comprised 21 (75.0%) of 28 histologically proven cases. There were three (10.7%) small cell carcinomas, one (3.6%) large cell carcinoma, and three (10.7%) adenocarcinomas. The proportion of Kreyberg group I in the occupational cases was significantly larger than that of lung cancers in the population-based cancer registries in Japan. These findings are compatible with Kreyberg's hypothesis. Above all, squamous and small cell carcinomas were prominent and appeared to be environmentally related bronchogenic carcinomas.