SummaryAlthough several studies have shown that the type and degree of unsaturation of fatty acids alter the production of proinflammatory mediators, there have been few studies in which the effects of trans isomers on allergic responses were evaluated. NC/Nga (NC) mice raised in conventional conditions develop spontaneous atopic dermatitis-like lesions with IgE hyperproduction. We used this model to evaluate the effect of dietary trans fatty acids (TFAs) on the development of spontaneous dermatitis. NC mice were fed a 4 g TFAs/kg diet (4 g TFA), a 8 g TFAs/kg diet (8 g TFA) or a control diet with no TFA content for 10 wk. The dermatitis condition improved with the increasing intake of TFAs, and there was a significant difference from week 5 of the experimental period between the dermatitis conditions in 8 g TFA-fed mice and control mice. Production of total IgE was also suppressed in mice fed TFAs compared with that in control mice, correlating with the skin lesions. IFN-␥ , but not IL-4, production was significantly greater in TFA-fed mice that in control mice. Our results suggest that intake of TFAs suppresses the development of atopic dermatitis-like lesions in NC mice. Even though we observed a decreased production of IgE levels, mechanisms involved in the development of dermatitis-like lesions seem to be IL-4-independent.