The consumption of n-3 fatty acids from seafood has been related to a lower incidence of coronary artery disease. Adipose tissue composition has served as a biological marker of chronic ingestion of many dietary polyunsaturated fatty acids. However, the incorporation of n-3 fatty acids into the fat depots has not been studied in humans. Daily dietary supplementation with > or = 10 g n-3 fatty acids from fish oil for > 12 mo resulted in significantly greater 20:5n-3, 22:5n-3, and 22:6n-3 concentrations in fatty acids of adipose tissue, and a greater 20: 5n-3 fatty acid content in plasma lipid classes (cholesterol esters, phospholipids, and free fatty acids) of supplemented subjects compared with nonsupplemented control subjects. Combined values for all subjects indicated that fatty acid concentrations of n-3 plasma lipid classes, including 20:5n-3, 22:5n-3, 22:6n-3, and total n-3, significantly correlated with corresponding concentrations of fatty acids in adipose tissue. These findings indicate that the long-term ingestion of large amounts of n-3 fatty acids in humans resulted in their incorporation into the adipose tissue fatty acids. Incorporation of the fatty acids into adipose tissue warrants consideration for use in clinical studies requiring precise documentation of long-term n-3 fatty acid consumption.