Suramin inhibits the growth of non-small cell lung cancer (NSCLC) and breast cancer in vitro by blocking the action of most known growth factors. The clinical efficacy of suramin was evaluated in patients with unresectable or relapsed NSCLC (n = 16) and advanced breast cancer (ABC) resistant to conventional therapies (n = 12). A plasma level > or = 200 micrograms/ml was maintained by three times weekly administrations using adaptive control with feedback. Treatment was continued until documented progression of disease or unacceptable toxicity. No clinical responses were observed in any patient. Median overall survival was 4.5 months in NSCLC and 9 months in ABC patients. Mean treatment duration was 6.6 weeks in NSCLC patients and 15.9 weeks in ABC patients. Treatment was discontinued due to disease progression in 14 patients, unacceptable adverse effects in 11 patients, while three patients refused to continue therapy. We cannot recommend this drug for further clinical trials in NSCLC and ABC.