Intravenous immunoglobulin (IVIG) concentrates were originally developed as replacement therapy for individuals with primary deficiencies of the immune system. However, in various well-designed, controlled clinical trials, the ability of IVIG to prevent and possibly treat infections in patients with secondary immune deficiencies has also been considered. In this review, we briefly consider these different applications and suggest whether the data are sufficient to employ IVIG in these clinical settings.