This paper presents the first case of asymptomatic IgG1 and IgG2 deficiency induced by carbamazepine (CBZ). The patient has remained asymptomatic since CBZ was started because IgM reduced only transiently, IgA decreased but remained within the normal range, and the specific antibodies to organisms having capsular polysaccharide antigens were not defective in this case. Analysis of membrane surface immunoglobulin (sIg) on B lymphocytes indicated that the maturation of B cells was defective from sIgM+ cells to sIgG+ cells. There have been no previous reports of IgG deficiency induced by CBZ due to this mechanism. Immunoglobulins should be carefully examined in patients treated with CBZ, particularly after its initiation.