The effects of dietary fat on carcinogenesis were presented, with the pancreas excluded, in randombred Syrian golden hamsters after administration of N-nitrosobis(2-oxopropyl)amine (BOP). Diets containing 4.5, 9, or 18 g corn oil/385 kilocalories [low-fat (LF), medium-fat (MF), or high-fat (HF) diet, respectively] were fed in two sequences. In the first sequence during which the effects of fat on the initiation phase of BOP carcinogenicity were examined, LF or HF diets were fed to hamsters 3-7 weeks of age and for 2 days after a single sc BOP treatment (10 mg/kg body wt) to 8-week-old hamsters. These hamsters were then given MF diet for the remainder of their lives. In the second sequence during which the role of fat on the promotional phase (development) of BOP-induced cancer was evaluated, MF diet was fed during the weeks preceding BOP treatment and LF or HF levels were given after BOP treatment. Separate groups were fed MF diet throughout both phases, and parallel animal groups received each diet sequence and were treated with saline at 8 weeks of age. Renal adenocarcinomas in males were observed only in those given HF diet either before or after BOP treatment (9% incidence). Similarly, pulmonary adenoma and intraphepatic biliary cystic adenoma (cholangioma) incidences were elevated above spontaneous rates in HF-fed groups. This study demonstrated that dietary fat enhanced BOP-induced tumorigenesis in the kidneys, lungs, and liver when fed, either during initiation (preceding carcinogen treatment) or at promotional stages (following carcinogen treatment).