This study assessed the choline status in newborns, infants, children, breast-feeding women, breast milk, infant forinula, breast-fed and formula-fed infants. The serum free choline level was 35.1 +/- 1.1 mu mol/L at birth and decreased to 24.2 +/- 1.6, 18.1 +/- 0.8, 16.3 +/- 0.9, 14.3 +/- 0.8, 12.9+0.6 or 10.9 +/- 0.6 mu mol/L at 22-28, 151-180, 331-365, 571-730, 731-1095 or 4016-4380 days after birth, respectively. The serum phospholipid-bound choline level was 1997 +/- 75 mu mol/L at birth and increased gradually to 2315 +/- 190 or 2572 +/- 100 mu mol/L at 571-730 or 4016-4380 days after birth, respectively. In breast-feeding women, serum free and phospholipid-bound choline levels were doubled at 12-28 days after birth, they decreased toward the control values with time. Free choline, phosphocholine and glycerophosphocholine were major choline compounds in breast milk. Their concentrations in mature milk were much greater than in colostrum and serum. Choline contents of breast milk varied greatly between mothers, and milk free choline levels were correlated with serum free choline (r.541; P <.001), phospholipid-bound choline (r=.527; P <.001) and glycerophosphocho line (r=.299-1 P <.01) concentrations and lactating days (r=.520 - P <.001). In breast-fed infants, serum free choline concentrations were correlated with free choline (r.47; P <.001), phosphocholine (r=.345; P <.002), glycerophosphocholine (r=.311; P <.01) and total choline (r=.306; P <.01) contents of breast milk. Serum free choline concentration in formula-fed infants was lower than breast-fed infants. These data show that (a) circulating choline status is elevated during infancy and lactation, (b) choline contents of breast milk vary between mothers and milk free choline contents are influenced by maternal circulating choline status, and (c) the choline contents of breast milk can influence infants' circulating choline status. (c) 2005 Elsevier Inc. All rights reserved.