The N-glycans present on the total mixture of serum glycoproteins (serum N-glycome) were analyzed in 24 subjects with congenital disorder of glycosylation type I (CDG-I) and 7 healthy, age-matched individuals. No new N-glycan structures were observed in the sera of CDG-I patients as compared with normal sera. However, we observed in all subtypes a significantly increased degree of core alpha-1,6-fucosylation of the biantennary glycans as compared to normal, as well as a significant decrease in the amount of triantennary glycans. These serum N-glycome changes appear to be a milder manifestation of some of the changes observed in adult liver cirrhosis patients, which is compatible with the reported steatosis and fibrosis in CDG-I patients. In the CDG-Ia subgroup, the extent of the serum N-glycome changes correlates with the aberration of the serum transferrin isoelectric focusing pattern, which measures the severity of the lack of entire N-glycan chains (primary consequence of CDG-I) in the liver and is the standard diagnostic test for this category of inherited diseases.