Earlier studies suggest that diet has no or only a minimal effect on serum lipoprotein(a) [Lp(a)] level. We compared serum Lp(a) concentration in infants being either breast-fed (only milk source was own mother's milk; n=202) or formula-fed (only milk source was infant formula; n=291) at the age of 7 months. Another group (n=98) received both breast milk and formula. Solid foods had been introduced to the diet of all children at the age of 3-5 months. The median Lp(a) concentration in breast-fed infants (24.5 mg/l) was 43% lower (P<0.001) than in formula-fed infants (43.0 mg/l). The median value in infants receiving both breast milk and formula was intermediate (34.5 mg/l). The difference between breast- and formula-fed infants was independent of the infants' relative weight and gender. The median of the mean Lp(a) concentration of the parents was similar in the two infant groups, suggesting that the difference in Lp(a) level between breast- and formula-fed infants was not due to genetic factors. The concentrations of serum cholesterol and apolipoprotein B were significantly higher in breast-fed infants compared to formula-fed infants. Our results show that breast-feeding is an important determinant of serum Lp(a) concentration in infancy and support earlier observations that serum Lp(a) and serum cholesterol levels are regulated largely independently of each other,