The fatty acid composition of serum lipids, erythrocytes, platelets, and diet was studied in women with insulin-dependent diabetes (IDDM) and in normal subjects matched for age, sex, body weight, and serum lipid levels. The dietary intake of linoleic acid was higher in IDDM patients than in the normal subjects. The linoleic acid content of serum triglycerides, cholesterol esters, and phospholipids and of red cells and platelets were elevated in patients with IDDM proportionately to their dietary linoleate intake. The linoleic acid content of serum lipids, but not of diet, was significantly correlated with glycosylated hemoglobin Alc in IDDM patients. However, the serum lipid contentof arachidonic acid and other n-6 polyunsaturated fatty acids, which are metabolites of linoleic acid, was decreased in IDDM patients, but these metabolites were normal or increased in their cell membranes. The contents of n-3 polyunsaturated fatty acids were decreased in serum and platelet lipids and tended to be increased in erythrocyte membranes of diabetic patients. The results suggest that elongation and desaturation of essential fatty acids, linoleic acid in particular, are decreased in women with IDDM.