Intravenous immunoglobulin (IVIG) replacement therapy is the mainstay of therapy for X linked agammaglobulinemia (XLA). This study was attempted to investigate how patients with XLA were treated with IVIG in Japan. Data were complied from questionnaires filled in by the physicians. One hundred eighteen medical records had been given from 134 patients. One hundred eleven patients had been treated with IVIG. Most patients had been administered IVIG every 2 to 4 weeks, and maintained serum IgG trough levels of 400 mg/dl. Some of patients had adverse effects of IVIG. Antibiotics, especially macrolides, had been administered in some of patients. A few patients, despite the maintenance of higher trough levels, were associated with infections. The present study suggests that IVIG should be administered dependent on personal infection histories.