The structure of over 93% of the sugar chains of serum transferrin purified from three patients with carbohydrate-deficient glycoprotein (CDG) syndrome was Neu5Ac alpha 2–>6Gal beta 1–>4GlcNAc beta 1–>2Man alpha 1–>6 (Neu5Ac alpha 2–>6Gal beta 1–>4GlcNAc beta 1–>2Man alpha 1–>3)Man beta 1–>4GlcNac beta 1–>4GlcNAc, similar to that in a healthy control. On chromatofocusing, CDG syndrome transferrin was separated into three major isoforms, S4, S2, and S0, containing 4, 2, and 0 sialic acids/molecule at pH 5.12 (5.16), 5.42, and 5.80, respectively. On 7.5% SDS-polyacrylamide gel electrophoresis, the molecular masses of transferrin isoforms S4, S2, and S0 were 80, 77, and 74 kDa, respectively. Transferrin isoforms S4 and S2 were linked to 2 and 1 mol of sialylated biantennary sugar chain/transferrin molecule, on the other hand, isoform S0 was not linked to any asparagine-N-linked oligosaccharide. Accordingly, CDG syndrome can be concluded to be an asparagine-N-linked oligosaccharide transfer deficiency, although the primary deficient enzyme has not yet been determined.