The aim of the study was to investigate the effect of age and feeding pattern on intestinal permeability during the first month of life. The subjects were 72 full-term, healthy neonates who were (a) exclusively breast-fed (BF group, n = 36) or (b) artificially fed (CM group, n = 36) with either an adapted formula (AF group, n = 17) or a partly hydrolyzed (hypoantigenic) formula (HA group, n = 19). A lactulose/mannitol (lac/man) intestinal permeability test was performed at 1 and 7 days (steady-state method, n = 72), then at 30 days of life (single oral load, n = 47). Urinary lactulose and mannitol were measured by HPLC. The mean lac/man urinary ratio dropped from 1.27 +/- 0.73 (day 1) to 0.34 +/- 0.36 at day 7 and to 0.22 +/- 0.21 at day 30. At 7 days BF infants showed a significantly lower lac/man urinary ratio (0.22 +/- 0.25) than the CM group (0.47 +/- 0.41). The human neonate shows a developmental pattern of sugar intestinal permeability that resembles gut closure observed in other mammals. Intestinal permeability decreases faster in breast-fed babies than in those fed with adapted or HA formulas.