Dihomo-gamma-linolenic acid (DGLA) is an n-6 polyunsaturated fatty acid that is mainly metabolized to an anti-inflammatory eicosanoid, prostaglandin (PG) E1, via the cyclooxygenase (COX) pathway. DGLA exists widely in the human body and daily animal-source foods. Concentrations of DGLA in the serum of atopic dermatitis patients are lower than those in healthy volunteers. DGLA suppressed clinical severity of skin lesions dose-dependently, with an increase in DGLA contents in phospholipids of skin, spleen, and plasma. Discontinuation of DGLA administration resulted in the onset of dermatitis and a decrease in DGLA contents in skin, spleen, and plasma. These findings indicate that oral administration of DGLA effectively prevents the development of atopic dermatitis in NC/Nga mice. DGLA may have an anti-atherosclerotic effect in apoE-deficient mice via PGE1 formation. As dihomo-γ-linolenic acid and arachidonic acid compete for processing by these oxidation enzymes, introduction of dihomo-γ-linolenic acid to platelets is correlated to suppression of arachidonic acid metabolites and promotion of dihomo-γ-linolenic acid metabolites such as PGE1, which produces an antithrombotic effect.