Human milk and colostrum contain ∼12-13 g/L and ∼22-24 g/L of oligosaccharides, respectively. The chemical structures of >100 human milk oligosaccharides (HMO) have been characterized to date. We determined the concentrations of 10 neutral and 9 acidic colostrum HMO collected during the first 3 d of lactation by using reverse phase HPLC after derivatization with 2-aminopyridine or 1-methyl-3-phenyl-5-pyrazolon. The predominant oligosaccharides were Fuc(α1-2)Gal(β1-4Glc (2'-FL), Fuc(α1-2)Gal(β1-3)GlcNAc(β1-3)Gal(β1-4)Glc (LNFP I), Fuc(α1-2)Gal(β1-3)[Fuc(α1-4)]GlcNAc(β1-3)Gal(β1-4)Glc (LNDFH I), and Gal(β1-3)GlcNAc(β1-3)Gal(β1-4)Glc (LNT), the concentration of each of which was ∼1-3 g/L. Because these HMO, other than 2'-FL, all contain the Lacto-N-biose type I structure [Gal(β1-3)GlcNAc], we conclude that HMO containing the type I structure predominate over those containing the N-acetyllactosamine type II structure [Gal(β1-4)GlcNAc]. This appears to be a feature that is specific to humans, because the milk and colostrum of other species, including apes and monkeys, either contain only type II oligosaccharides or type II predominate over type I. It is possible that type I HMO may have importance as substrates for beneficial bifidobacteria in breast-fed infants. The biological importance of type I HMO predominance warrants further study, both in relation to human health and to human evolution.