Carbohydrate-deficient glycoprotein syndrome type I (CDGS I) is an autosomal recessive disease with multiple organ manifestations. The diagnostic biochemical marker has been typical carbohydrate-deficient isoforms of transferrin (Tf). Many other glycoproteins in blood may show similar defects, but have not been systematically studied before. Forty-eight CDGS I patients and 22 controls were examined for total concentrations and isoform distribution of Tf, antithrombin (AT), α1-antitrypsin (α1-AT) and thyroxine-binding globulin (TBG), and for the level of carbohydrate-deficient transferrin (CDT). The absolute values varied with age. The most frequent persistent quantitative changes were reduced levels of AT (97%) and elevated CDT values (100%). Isoforms lacking one to eight of four to eight possible sialic acid residues were found in AT, TBG and Tf in all cases, with variable intensity and frequency, and in all except one patient in α1-AT. The isoform changes were most constant and pronounced in Tf. The other three glycoproteins showed more abnormal heterogeneity in the youngest than in the older patients. The results indicated that the biochemical defect stabilizes with age, and suggested partial hypoglycosylation rather than nonglycosylation of these glycoproteins. Analysis of Tf isoforms is still the safest diagnostic marker of CDGS I from full-term birth and over the ages.