Lonidamine, a substituted indazole carboxylic acid with unique effects on cellular respiration, was studied in 27 patients with advanced malignancies. Of the 18 evaluable patients, 5 had small-cell lung cancer, 3 had non-small-cell lung cancer, 3 sarcoma, 2 breast cancer, and 5 other tumour types. All but 1 had had extensive prior treatment. A partial response was seen in 1 patient with metastatic synovial sarcoma, and tumour growth inhibition was demonstrated in 2 other cases. The major toxicity encountered was myalgia (66.6%) which was incompletely ameliorated by prednisone and required dose reduction in 2 patients and cessation of drug in 3. Other toxicities included auditory changes, anorexia, nausea and vomiting, diarrhoea, skin sensitivity, and conjunctivitis. No added toxicity was seen, when Lonidamine was combined with other chemotherapeutic agents. No correlation between Lonidamine dose and serum lactate levels was seen, although 4 patients showed a progressive increase in lactate levels over time, thought to be related to their increasing tumour burden. 5 patients demonstrated a dramatic fall in serum testosterone levels 4-8 weeks after starting Lonidamine which was accompanied by an increase in luteinizing hormone levels in 3 patients. In summary, modest antitumour activity was demonstrated in 3 patients; moderate toxicity was seen in most patients, but was usually tolerable. Further studies of Lonidamine are warranted in less heavily treated patients, alone or in combination with other chemotherapeutic agents.