Intravenous immunoglobulin (IVIg) is a safe preparation made from human plasma. The original concept of IVIg as an anticancer drug was built up over the years, after numeral reports were accumulated indicating cancer regressions after IVIg administration. Because IVIg is basically an established remedy for immunodeficiencies and several autoimmune diseases, the association between beneficial effects in cancer patients after IVIg was first seen in patients who had both cancer and autoimmune or immunodeficiency diseases. Interestingly, cancer and autoimmunity share several common features, which together enhance the notion of using IVIg to treat cancer. Several studies tested the broad range of the antimetastatic effects of IVIg. IVIg was found to operate in many different and complex ways, among them (a) induction of interleukin-12 secretion, leading to natural-killer-cell activation; (b) inhibition of matrix metalloproteinase-9 mRNA expression; (c) suppression of tumor cell growth; (d) hindrance of nuclear factor κB activation and IκB degradation; and (e) G1 cell-cycle arrest. In conclusion, IVIg is a potential anticancer treatment for several reasons: (a) the bidirectional relationship between cancer and autoimmunity; (b) the apparent association between cancer regression and IVIg administration; (c) a variety of anticancer effects of IVIg observed; and (d) IVIg is considered to be a safe preparation with minimal side effects. Obviously, prospective controlled studies that will establish the antitumor effects of IVIg are needed.