The influences of the maternal diet and of the mothers' nutritional and socioeconomic status on the fatty acid composition of human milk have not yet been fully elucidated. Fatty acids from capric (8:0) to docosahexaenoic (22:6w3) acids were determined in 209 samples of human milk obtained from voluntary donors. Samples were classified according to the time of lactation and in relation to the socioeconomic status and to the weight of the lactating women. Capric and lauric acid increased in mature milk while palmitic and stearic acids decreased. Polyunsaturated fatty acids (PUFA) of the w6 and w3 series with more than 18 carbon atoms also decreased from colostrum to mature milk. However, the ratio w3 PUFA/w6 PUFA remained unaltered. Oleic acid decreased in milk from mothers of a medium and low socioeconomic status who consumed almost exclusively seed vegetable oils. The mother's weight did not influence the fatty acid composition of her milk.