To evaluate changes that occur in serum cholesterol ester fatty acid composition during the transition from typical infant feeding to a more adult type of nutrition, this study compared the effects on serum cholesterol ester fatty acids of breast milk or formula at the age of 7 mo with effects caused by 6-mo dietary intervention in 137 children. The intervention [Special Turku coronary Risk factor Intervention Project for children (STRIP baby project)] aimed at a reduction of saturated fat intake to 10% of energy after the age of 1 y without purposefully influencing total fat intake. Nutrient intakes were calculated from 3-d food records. At the age of 7 mo, i.e. before dietary education began, milk type markedly influenced dietary and serum cholesterol ester fatty acid composition (mean serum cholesterol ester 16:0 in breastfed vs formula-fed infants, 13.7% vs 12.0%, respectively, p < 0.001; serum cholesterol ester 18:2n-6 50.6% vs 57.6%, p < 0.001). At the age of 13 mo the calculated fat intake of the intervention and control children differed markedly but serum cholesterol ester fatty acid compositions in all children resembled closely those measured in 7-mo-old breastfed infants, e.g. at the age of 13 mo the relative proportions of 18:2n-6 were 49.9% and 51.1% in previously formula-fed intervention and control children, respectively, and 50.3% and 50.1% in previously breastfed intervention and control children, respectively. In conclusion, serum cholesterol ester fatty acid composition reflected differences in dietary fat quality (breast milk or formula) at the age of 7 mo, whereas dietary intervention as applied in the STRIP baby project had only a minimal effect.