The effects of dietary supplementation with fish on plasma fatty acid levels were studied in 20 coronary heart disease patients who had suffered acute myocardial infarction. The study was divided into three periods: hospital admission, after 8 weeks on a heart-healthy diet designed for patients with ischemic heart disease, and after 4 weeks on an n-3 fatty acid-supplemented diet in which red meat was replaced with lean and fatty fish. At the end of each period, the subjects responded to a 48-hour recall questionnaire, so that we could assess their compliance with the diet, and blood samples were collected for the determination of plasma fatty acids. Stearic fatty acid was significantly decreased after the fish diet. n-7 and n-9 fatty acids showed no significant changes throughout the study. At the end of the 4-week period when the fish diet was consumed, linoleic acid and its long-chain derivative docosapentaenoic acid (22:5 n-6) were significantly increased. The most notable changes in n-3 series fatty acids at the end of the third period were the significant decrease in linolenic acid and the significant increase in its long-chain derivatives eicosapentaenoic (20:5 n-3) and docosahexaneoic acid (22:6 n-3). These changes in plasma fatty acid levels may have beneficial effects on coronary heart disease.