alpha-Linolenic acid deficiency is described in three patients. Observed clinical symptoms were hemorrhagic dermatitis, hemorrhagic folliculitis, skin atrophy, and scaly dermatitis. Supplementation with ethyl alpha-linolenate followed by a purified fish oil (EPA-oil) began to normalize symptoms within 10 d. The mitogenic response in isolated lymphocytes was reduced whereas the number of T lymphocytes increased significantly. Serum thromboxanes, urinary excretion of 2,3-dinor-6-keto-prostaglandin F1 alpha (PGI2-M), and bleeding time were unaffected. The results indicate that omega-3 fatty acids are essential for normal accumulation of erythrocyte omega-6 acids. The dietary intake of long-chain omega-3 acids required to obtain midnormal concentrations of omega-3 acids in plasma and erythrocyte lipids was estimated to be 350-400 mg/d (0.4% of calories), whereas the corresponding mean intake of alpha-linolenic acid was 990 mg/d (1.0% of calories). It is suggested that essential fatty acid requirement should be stated as grams or milligrams per day, similarly to other essential nutrients.