This report describes a study by Dr. Moon and colleagues to investigate the carcinogenic potential of 1-nitropyrene, a mutagenic constituent of diesel exhaust particles, using a hamster respiratory-carcinogenesis model. Male hamsters were exposed to 1 or 2 mg of 1-nitropyrene via intratracheal administration either once or twice a week for 92 weeks. In order to study activity as a cocarcinogen, 1 or 2 mg of 1-nitropyrene was administered in combination with 0.25 mg of the known environmental carcinogen benzo[α]pyrene once per week for 92 weeks. Body weight, survival, and lung and trachea tumor incidence were measured.