Risk for coronary heart disease is increased in adult Caucasians with high serum lipoprotein(a) [Lp(a)] concentration. In adults the concentration is mainly regulated by genetic factors. Our previous study suggests that breast milk has a beneficial effect on serum Lp(a) concentration in infants. Now we analyzed the influence of weaning by measuring serum Lp(a) and cholesterol in 414 infants at 7, 13, 24, and 36 mo of age. At 7 mo the infants received, in addition to solid food, only breast milk (n = 148), breast milk and formula (n = 74), or formula only (n = 191). Median (range) serum Lp(a) concentrations were then 25 (< or = 12-743) mg/L, 35 (< or = 12-1188) mg/L, and 45 (< or = 12-577) mg/L in the three feeding groups, respectively (p = 0.0013). Breast milk and formula were changed to cow's milk in all infants before 12 mo of age. At 13 mo serum Lp(a) concentration had increased more in infants who were weaned from breast milk than in those who had been fed both breast milk and formula, or formula only (median increases 37, 26, and 20 mg/L, respectively; p = 0.0062). Thus the serum Lp(a) concentration was similar in all feeding groups at 13 mo. This finding was also observed at 24 and 36 mo. The increase in serum Lp(a) concentration was independent of the baseline Lp(a) level, apolipoprotein E phenotype, gender, and weight gain of the infants between 7 and 13 mo.(ABSTRACT TRUNCATED AT 250 WORDS)