AbstractAmount and type of fat and energy density of diets may influence tumor incidences. The purpose of this report is to summarize the influence of com oil gavage and different nonpurified diets on spontaneous tumor incidences in 64 diet and 59 com oil gavage control groups in two‐year studies involving approximately 6,100 control Fischer 344 rats of each sex. The maximum mean body weight attained during the course of the study, survival at 106 weeks of age, and spontaneous tumor incidences of groups fed different nonpurified diets with or without corn oil gavage were summarized and evaluated for differences. Male rats fed NIH‐07 open‐formula diet with or without corn oil gavage had significantly higher body weight, lower survival, and higher incidence of pancreatic acinar cell tumors than rats fed commercial proprietary diets with or without corn oil gavage. Female rats fed NIH‐07 diet with or without corn oil gavage had significantly higher body weights and pancreatic tumor incidences than groups fed commercial diets. Time‐related trends could account for other apparent differences in tumor incidences between the groups fed commercial and NIH‐07 diets. Corn oil gavage significantly increased the body weight and pancreatic tumor incidences but decreased the incidence of leukemia, a lethal tumor, which resulted in higher survival in male rats. Corn oil gavage significantly lowered the body weight and anterior pituitary tumor incidence in female rats. The pancreatic acinar cell tumor incidence appears to be due to a combination of fat intake and body weight.