The effect of feeding with human milk and commercially available milk substitutes was studied in a group of 154 healthy infants during the first 3 months of life by assessment of body weight, body length, head circumference, skinfold thickness, serum lipid and lipoprotein concentrations. Human milk and the different milk formulae have the same energy content (kcal/100 ml) and total fat, total protein and total carbohydrate contents are comparable but they differ in respect of their fatty acid compositions. The various diets were chosen freely by the parents and the newborns were exclusively fed either human milk (n = 56), Multival 1 (n = 31), Humana 1 (n = 33), or Pre Aptamil (n = 34). No significant differences in body weight, body length or head circumference were observed between any of the different dietary groups. Fat storage, as assessed by measurements of skinfold thickness, was significantly less in breast-fed children compared to those on the formula diets. Breast-fed and Pre Aptamil-fed infants had the highest levels of total serum cholesterol, low density lipoprotein (LDL)-cholesterol and LDL. No differences were observed in the levels of total serum triglycerides, very low density lipoprotein (VLDL)-and high density lipoproteins (HDL)-cholesterol, VLDL and HDL. There were no strong correlations between the physical and the biochemical parameters. No indication of an increased risk of developing atheroselerosis was associated with any of the dietary treatments for the duration of this study. However, these investigations support the hypothesis that subtle early nutritional variation can influence mechanisms that regulate lipoprotein and cholesterol levels in later life.