The data of 29 patients with X-linked agammaglobulinemia, who received immunoglobulin replacement therapy between 1965 and 1990, were analyzed for dose-dependent long-term results concerning infectious complications. Patients who received high-dose intravenous immunoglobulin replacement (greater than 400 mg/kg every 3 weeks) showed a significant increase in trough serum IgG levels and a significant decrease in the incidence of pneumonias and the number of days spent in the hospital compared with patients receiving intravenous immunoglobulin low-dose (less than 200 mg/kg every 3 weeks) or intramuscular immunoglobulin (less than 100 mg/kg every 3 weeks) treatment. Improvements in therapeutic outcome were particularly evident when high-dose intravenous immunoglobulin replacement therapy was started before the age of 5 years. Bacterial meningitis, chronic pulmonary disease, and bronchiectasis occurred in the intramuscular immunoglobulin group but did not occur in either of the intravenous immunoglobulin groups. High-dose intravenous immunoglobulin therapy may have a positive impact on the clinical course and may prevent severe complications in patients with X-linked agammaglobulinemia.