This paper reviews the use of low-dose cyclophosphamide (CY) with active specific immunotherapy in patients with advanced melanoma and other metastatic cancers, and outlines the basic scientific research that supports this use. In various animal models, CY augments delayed-type hypersensitivity responses, increases antibody production, abrogates tolerance, and potentiates antitumor immunity. The mechanism of CY immunopotentiation involves inhibition of a suppressor function, as indicated by extensive work in the MOPC-315 plasmacytoma murine model. Human studies of the immunopotentiating effect of CY have yielded both positive and negative results. Toxicity associated with low-dose CY has been mild in these studies. Results of efficacy have been variable for reasons such as small sample sizes, short follow-up periods, and the weaker immunogenicity of human tumor-associated antigens. Although beneficial clinical outcomes have been observed in historically controlled trials, there are few randomized, controlled trials that evaluate outcome in relation to CY immunopotentiation of active specific immunotherapy. Additional randomized, controlled trials should be done to examine the clinical efficacy of CY immunopotentiation of therapeutic cancer vaccines.