Lung cancers were induced in inbred W rats by cadmium chloride aerosols. For 18 months, 120 male W rats were continuously exposed to cadmium chloride aerosols with cadmium (Cd) concentrations of 12.5, 25, and 50 micrograms/m3, respectively. For the same period of time, 41 rats were kept in filtered air; these rats served as the control group. The survivors were killed 13 months after the end of the inhalation experiments. Histopathologic examination revealed a dose-dependent incidence of primary lung carcinomas of the following types: adenocarcinomas, epidermoid (squamous cell) carcinomas, combined epidermoid carcinomas and adenocarcinomas, and mucoepidermoid carcinomas. The incidence of lung carcinomas was 71.4% in the group exposed to 50 micrograms Cd/m3, 52.6% in the group exposed to 25 micrograms Cd/m3, and 15.4% in the group exposed to 12.5 micrograms Cd/m3. None of the controls developed lung carcinomas. At the end of the experiment, the remaining Cd concentrations in the lungs were relatively high, almost at the same level as those in the livers.