The purpose of the study was to evaluate the safety, efficacy, and pharmacokinetics of Flebogamma® 5%, an immune globulin intravenous product, for replacement therapy in primary immunodeficient patients. The US Food and Drug Administration has proposed that the use of new products must result in ≤ 1 serious bacterial infection/subject/year, have acceptable safety and tolerability, and have pharmacokinetic properties similar to endogenous IgG and other commercially available immune globulin products. Flebogamma® 5% was administered at seven clinical sites to 51 subjects aged 14–74 years with well-defined primary immunodeficiency diseases at a dose of 300–600 mg/kg every 21–28 days for 12 months. The calculated serious infection rate for the intent-to-treat population was 0.061/subject/year. The incidence of adverse events considered potentially related to Flebogamma® 5%, and occurring during or within 72 h after completing the infusion was approximately 8%. The half-life of total IgG was 37 days. Flebogamma® 5% is efficacious, safe, and well-tolerated, and does not put subjects at increased risk of adverse events other than those that could be reasonably expected in primary immunodeficient subjects who are receiving any immune globulin product.