The fatty acid composition of erythrocytes, of plasma choline phosphoglycerides, and of adipose tissue, serum cholesterol, triglyceride and vitamin B12 concentrations, weights, heights and skinfold thickness were determined on 22 vegans and 22 age and sex matched omnivore controls. The fatty acid composition of breast milk from four vegan and four omnivore control mothers, and of erythrocytes from three infants breast fed by vegan mothers and six infants breast fed by omnivore control mothers was determined. The proportions of linoleic acid and its long-chain derivatives were higher, the proportion of the long-chain derivatives of alpha-linolenic acid was lower, and the ratio of 22:5omega3/22:6omega3 was greater in the tissues of the vegans and infants breast-fed by vegans than in controls; the most marked differences were in the proportions of linoleic (18:2omega6) and docosahexenoic (22:6omega3) acids. Weights, skinfold thickness, serum vitamin B12, cholesterol and triglyceride concentrations were less in vegans than in controls. The difference in serum cholesterol concentration was most marked. It is concluded that a vegan-type diet may be the one of choice in the treatment of ischemic heart disease, angina pectoris, and certain hyperlipidemias.