Pericardial effusions were found in 6 of 10 children with carbohydrate-deficient glycoprotein syndrome type I (CDGS-I). In three cases pericardectomy was necessary. Blood concentrations of several glycoproteins and albumin were low. Similar abnormal isoforms of four glycoproteins were found in blood (B) and pericardial fluid (PF). There was a significant negative correlation between the mean concentration ratio PF/B and the molecular mass (MW) of 11 proteins. For proteins with MW < 100 kDa there were significant correlations in the controls, but not in the patients, between the PF/B ratio and both the MW and the sialic acid contents in the (glyco-)proteins. The pericardium exhibited focal mixed inflammatory changes with mesothelial proliferation, with widened endoplasmic reticulum and flocculent and/or lamellated material. Damage to a pericardial protein barrier is suggested to be involved in pericardial effusion in CDGS-I.