The promoting effects of diets varying both in type and amount of fat on N-nitrosomethylurea [(NMU) CAS: 684-93-5]-induced mammary tumorigenesis were assessed in female inbred F344 rats. Two seed oils (safflower and corn) and two fruit oils (olive and coconut), varying widely in their diene, monoene, and saturated fatty acid ratios, were fed in the casein-based AIN-76A diets at 23% [high-fat (HF) diet] and 5% [low-fat (LF) diet] by weight, with the exception of coconut oil which was fed only at 23%. The predominant fatty acid in safflower and corn oils was linoleic acid (82 and 56%, respectively), while the predominant fatty acids in olive and coconut oils were oleic (79%) and myristic (54%), respectively. The test diets were fed beginning 2 days after administration of NMU and continued until termination of the experiment at 22 weeks post NMU administration. Analysis of tumor incidence, latency, and multiplicity data obtained from the 7 experimental groups indicated that animals fed the HF safflower and corn oil diets exhibited enhanced mammary tumor yields when compared to animals fed HF olive or coconut oil diets or their LF counterparts. Since weight gains and total caloric intake were similar in all 4 HF groups, the results of this study indicate that the tumor-promoting properties of HF diets are more of a function of differences in fatty acid composition than of fat content per se or of total caloric intake.