IgG subclass levels were measured before and after administration of intravenous immunoglobulins (IVIGs) in patients with common variable immunodeficiency (CVI). Six patients were treated with IVIG at the dose of 100-150mg/kg to maintain a trough level of 200mg/dl every 4 or 5 weeks, except for patient 5 who was given IVIG only 3 or 4 times per year. Three kinds of IVIGs, polyethylene-glycol (PEG)-treated IVIG, alkylated IVIG and sulfonated IVIG were used for replacement therapy. Although serum IgGl levels were low before administration of IVIGs, they increased to the normal levels after each administration of IVIGs in four patients. IgG2 preserved normal levels before and after administration of IVIGs in all patients. IgG3 was present at low normal concentration in patient 1, low concentration in patients 2, 3 and 6, and undetectable in patients 4 and 5 before infusion. Although increases in IgG3 levels were shown after infusion of PEG-treated IVIG, there were no increases after infusion of sulfonated or alkylated IVIG. However, there have been several reports that IgG3 is detected in sulfonated or alkylated IVIG preparations by another method. IgG4 levels were somewhat low before administration, but four patients achieved normal serum levels with treatment. In light of the above results of replacement therapy with IVIGs, we should consider the IgG subclass levels for patients such as CVI or selective IgG subclass deficiency.