The effect of habitual maternal diet on the fatty acid composition of breast milk was evaluated by conducting a dietary survey of lactating mothers and by analyzing their breast milk samples. The mean energy intake of the mothers was 9.8 MJ during the first survey week (20 mothers, 6-8 weeks post partum) and 8.6 MJ during the second survey week (13 mothers, 17-22 weeks post partum). Protein, fat and carbohydrate intake comprised 16%, 39% and 45% of the total energy intake, respectively. The breast milk samples contained 3.8% fat. Half of the fatty acids of the diets and breast milk were saturated (S), and one third were monoenoic. Polyunsaturated (P) fatty acids comprised 15% in the diets and 13% in the breast milk. The average P/S ratio was 0.3 both in the diets and breast milk samples. Carbohydrate intake correlated negatively (p less than 0.05) with arachidic acid of breast milk, and both protein and fat intakes correlated positively (p less than 0.05) with palmitic acid of breast milk. The maternal diet had no effect on the total fat content of breast milk. A positive correlation was found between the saturated (p less than 0.05) and polyunsaturated (p less than 0.01) fatty acids of maternal diet and breast milk. As to the single fatty acids, the only significant correlation was found with linoleic acid (p less than 0.01). However, the variation of linoleic acid intake explained only about 27% of the variation occurring in the breast milk.