Antiidiotypic antibodies (Ab2) have been shown to induce specific immunity against murine and human tumors. Studies in murine tumor systems also have shown that Ab2 immunizations can induce protective immunity against a challenge with tumor cells. Antibodies and cytotoxic/regulatory T cells have been implicated as mediators of the antitumor effects induced by Ab2 immunization in mice. In recent clinical trials, cancer patients immunized with Ab2 that functionally mimic a gastrointestinal tumor-associated antigen developed highly specific antitumor immune responses. Thus Ab2 are promising vaccine candidates in immunotherapy approaches to cancer. The choice of Ab2 as well as optimization of immunization procedures with Ab2 are important considerations in future studies.