The effects of two anti-thrombotic and anti-lipidemic oils, evening primrose oil and fish oil, on glucose and lipid metabolism, prostaglandin (PG) levels and body composition were studied in patients with non-insulin-dependent diabetes. Seven patients were administered 4 g evening primrose oil, 2.4 g sardine oil and 200 mg vitamin E for 4 weeks. Fasting plasma glucose, hemoglobin A1c, total cholesterol, body weight and % body fat mass were significantly decreased after the treatment, and levels of changes in these parameters were not different from 11 patients who did not receive the oils. In the treatment group, concentrations of (e) icosapentaenoic acid (EPA) increased significantly in all the lipoprotein fractions, but dihomo-gamma-linolenic acid (DGLA) increased only in the high-density lipoprotein (HDL) fraction. The treatment decreased urinary 11-dehydro-thromboxane B2 excretion (32.7% decrease, P < 0.05), but did not alter significantly plasma PGE1 or 6-keto-PGF1 alpha levels. The ratio of 6-keto-PGF1 alpha and PGE1 to 11-dehydro-thromboxane B2 increased significantly after the treatment. These results suggest that these oil treatments are useful in improving abnormal lipid and thromboxane (TX)A2 metabolism in diabetic patients.