15 patients with metastatic cancer were treated with Lonidamine, a substituted indazole carboxylic acid active against the Lewis lung and Sarcoma 180 tumors. Single doses of 600 mg (350-400 mg/m2) mostly induced somnolence and gastro-intestinal side effects. Toxicity occurring during chronic administration of Lonidamine at doses ranging between 45 and 275 mg/m2 twice daily, mainly consisted of somnolence, myalgias, hyperesthesia and mild hair loss. Myalgias and hyperesthesias were markedly relieved with prednisone 5 mg twice daily. No laboratory abnormalities were seen. In 1 patient with breast cancer resistant to standard chemotherapeutic agents, a 30% reduction of measurable tumor masses was seen. In view of the lack of overlapping toxicity between Lonidamine and other chemotherapeutic drugs, and of its potential activity, it is suggested that phase II studies of Lonidamine be initiated at a dose of 135 mg/m2 twice daily.