Primary immunodeficiency diseases are not common in children. The possibility of an immunological defect should be considered in any individual with repeated infections. A definite diagnosis for immodeficiency is sometimes difficult to achieve because of overlapping clinical manifestations. Immunoglobulin subclass deficiency is an immunological deficiency disease with which, one or more IgG subclasses are deficient. T cell immunity is normal. Patients may develop recurrent bacterial and respiratory infections or could remain asymptomatic. OBJECTIVE The authors report a case of immunoglobulin G subclass deficiency presenting initially as transient hypogammaglobulinemia of infancy. METHODS A 2 month-old boy presented to Siriraj Hospital with a history of chronic protracted diarrhea, disseminated scabies and sepsis. On presentation, he had generalized scaly and maculopapular rash with no palpable lymph nodes. CBC revealed WBC 22,100 cells/cm3 with PMN 42 per cent, lymphocytes 38 per cent, Eosinophils 4 per cent, Basophil 2 per cent and platelets 254,000/cm3. The immunoglobulin levels were as follows: IgG 181 mg/dl, IgA < 6.6 mg/dl, IgM 26.3 mg/dl. Lymphocyte enumerations revealed CD4 of 2,433 cells/cm3 (N 1,460-5,160); CD8 4,682 cells/cm3 (N 650-2,450); CD19 1,588 cell/cm3 (N 500-1,500); CD16 230 cell/cm3 (N 573 +/- 264). The initial diagnosis was X-linked agammaglobulinemia vs common variable immunodeficiency disease. His diarrhea and five courses of sepsis responded well to antibiotics administration and courses of intravenous immunoglobulin (IVIG) replacement. His through IgG became normal at 2 years of age (after 12 months of IVIG). IVIG was stopped and the diagnosis was changed to transient hypogammaglobulinemia of infancy (THI). Nevertheless, during his 4 month follow-up he developed recurrent sinopulmonary infections (i.e, otitis media and pneumonia). Repeated immunoglobulin profile showed IgG 1,200 mg/dl, IgA 135 mg/dl, IgM 26 mg/dl, IgG subclass were IgG, 1,030 mg/dl (N 280-830), IgG2 30 mg/dl (N 40-2,400), IgG3 22 mg/dl (N 6-130), IgG4 3 mg/dl (N 3-120). A diagnosis of IgG2 subclass deficiency presenting early as transient hypogammaglobulinemia of infancy was then made. Treatment with monthly IVIG was reinitiated and the patient is currently doing well. CONCLUSIONS The authors present a case of IgG subclass deficiency presenting as transient hypogammaglbulinemia of infancy. Follow-up of the immune profile and clinical manifestation is necessary for a definite diagnosis.