Congenital disorder of glycosylation (CDG), formerly representing a group of 
diseases due to defects in the biosynthetic pathway of protein N-glycosylation, 
currently covers a wide range of disorders affecting glycoconjugates. Since its 
first application to serum transferrin from a CDG patient with 
phosphomannomutase-2 deficiency in 1992, mass spectrometry (MS) has been playing 
a key role in identification and characterization of glycosylation defects 
affecting glycoproteins. MS of native transferrin detects a lack of glycans 
characteristic to the classical CDG-I type of molecular abnormality. 
Electrospray ionization MS of native transferrin, especially, allows glycoforms 
to be analyzed precisely but requires basic knowledge regarding deconvolution of 
multiply-charged ions which may generate ghost signals upon transformation into 
a singly-charged form. MS of glycopeptides from tryptic digestion of transferrin 
delineates site-specific glycoforms and reveals a delicate balance of 
donor/acceptor substrates or the conformational effect of nascent proteins in 
cells. Matrix-assisted laser desorption ionization MS of apolipoprotein C-III is 
a simple method of elucidating the profiles of mucin-type core 1 O-glycans 
including site occupancy and glycoforms. In this technological review, the 
principle and pitfalls of MS for CDG are discussed and mass spectra of various 
types of CDG are presented.