We administered suramin, an anti-parasitic drug and reverse transcriptase inhibitor, to 15 patients with metastatic cancer. This compound is known to inhibit the binding of growth factors (eg, epidermal growth factor [EGF], platelet-derived growth factor [PDGF], tumor growth factor-beta [TGF-beta]) to their receptors and thus antagonize the ability of these factors to stimulate growth of tumor cells in vitro. There were no complete responses (CRs), four partial responses (PRs) (two of ten adrenal cortex, one of four renal, one of one adult T-cell leukemia-lymphoma [HTLV-1]), and two minimal responses (MRs) (two of ten adrenal cortex). Toxicity included proteinuria (14 patients), reversible liver function test abnormalities (eight), vortex keratopathy (five), adrenal insufficiency (three), coagulopathy secondary to increased circulating levels of glycosaminoglycans (11), and one case of a reversible acute demyelinating polyneuropathy resembling the Guillain-Barrè syndrome. We conclude that suramin is an active agent in the treatment of metastatic cancer, and further work is necessary to define its scope.