The effects of dietary lipid level, degree of saturation, and antioxidant supplements on ultraviolet-light (UV) carcinogenesis were studied in female albino hairless mice. Twelve groups of 42 animals each received a restricted, semipurified, isocaloric diet containing 4%, 12%, or 12% (60% hydrogenated) corn oil with or without antioxidants (2%, w/w). A regimen of escalating UV irradiation was employed until a cumulative dose of 142 J/cm2 had been delivered. Tumor development time in 50% of the population (TDT50) was derived from a cumulative distribution of time to tumor formation, which was estimated for all groups. Although there were no significant differences in TDT50s between animals receiving low and high unsaturated lipid dietary regimens, animals receiving hydrogenated corn oil demonstrated a significantly (p less than 0.01) greater TDT50 and fewer tumors per animal than those receiving either level of unsaturated corn oil. Antioxidants had no effect on TDT50s within any of the dietary groups. However, greater tumor multiplicity was observed in groups receiving unsaturated lipid and antioxidants. These data demonstrate that the degree of dietary lipid saturation modifies the carcinogenic response to UV and suggest that dietary lipid may modify the previously reported inhibitory effect of antioxidants on UV carcinogenesis. It may be concluded that adherence to dietary standards is as important as other experimental parameters when comparisons of UV effects are involved.