We are reporting the first case of oxcarbazepine-induced immunoglobulin deficiency. Although this is known to be a rare adverse reaction to various pharmacologics (1,4,5), including the anticonvulsant carbamazepine (3, 6, 10, 11), it has not been previously described with oxcarbazepine.A 49-year-old white female was referred for further investigation of low serum immunoglobulins found as part of an evaluation for chronic bacterial vaginitis and suspected immune deficiency. Her infectious history was significant for one episode of pneumonia, several episodes of sinusitis, and chronic bladder infections.As a part of her immunologic evaluation, causes of secondary hypogammaglobulinemia were considered. She had no evidence of gastrointestinal protein loss and no evidence of malignancy and was not taking any immunosuppressive medications. She had been given oxcarbazepine (1,800-mg total daily dose), a derivative of the anticonvulsant carbamazepine, for chronic pain.At referral, immunoglobulin levels were low (immunoglobulin G [IgG], 576 mg/dl; IgA, Ͻ11 mg/dl; IgM, Ͻ4 mg/dl) and she had a B-cell deficiency (1%, 18 B cells/mm 3 [normal, 5 to 15%, 75 to 375 B cells/mm 3 ]). She maintained positive (immune) IgG responses to measles, rubella, mumps, tetanus, and diphtheria. Antibody responses 1 month after pneumococcal vaccination were poor (protective antibody, 1.3 g/ml, to only 2 out of 12 serotypes tested).Suspecting a potential adverse reaction to oxcarbazepine, this medication was discontinued; immunoglobulin levels and B-cell numbers remained low for at least 2 months. Her IgG and IgM levels returned to normal after 8 months (IgG, 977 mg/dl; IgA, Ͻ7 mg/dl; IgM, 62 mg/dl), with normalized B-cell numbers (5%; 85 B cells/mm 3 ), and remained normal at 12 months (IgG, 1,065 mg/dl; IgA, Ͻ15 mg/dl; IgM, 64 mg/dl; 113 B cells/mm 3 [7%]), with protective antibody responses to 5 out of 12 pneumococcal serotypes ( Fig. 1 and 2). She continued to have IgA deficiency. Her chronic pain returned with the discontinuation of oxcarbazepine.