Polyunsaturated fatty acids (PUFA) have been used to treat dogs with atopic dermatitis but the mechanism of action has not been well understood. The aim of this study was to evaluate the in vitro influence of PUFA on canine peripheral blood mononuclear cells (PBMC). PBMC isolated from eleven dogs with atopic dermatitis and eleven healthy control dogs were stimulated with concanavalin A and Dermatophagoides farinae extract in the presence of linoleic acid (LA), gamma-linolenic acid (GLA), alpha-linolenic acid (ALA), eicosapentaenoic acid (EPA)/docosahexaenoic acid (DHA) and GLA/EPA/DHA. Subsequently, quantitative polymerase chain reaction (qPCR) for interferon (IFN)-gamma, interleukin (IL)-4 and transforming growth factor (TGF)-beta m-RNA was performed. In the presence of concanavalin A, only PBMC of healthy dogs showed a gradual reduction in proliferation index from incubation without PUFA to incubation with ALA, EPA/DHA and GLA/EPA/DHA, respectively. A similar reduction was seen in normal and in atopic dogs in the presence of D. farinae allergen after incubation with ALA, EPA/DHA and GLA/EPA/DHA. In both groups IL-4 and IFN-gamma but not TGF-beta gene transcription was upregulated, when cells were incubated with D. farinae. Allergen-induced upregulation was not influenced by incubation with PUFA. These findings suggest that PUFA are able to influence proliferation of peripheral blood mononuclear cells in healthy and atopic dogs but do not seem to influence gene transcription of IL-4, IFN-gamma and TGF-beta.