A total of 34 patients with advanced cancer were given weekly or daily escalating doses of oral gossypol, a cottonseed-oil constituent showing evidence of antineoplastic activity in pre-clinical studies. No major adverse events occurred and there was no evidence of haematological or biochemical disturbance. As determined by dose escalation in 17 patients, the dose-limiting toxicity was emesis in 16 patients. There was no evidence of tumour regression in any of the 20 patients assessed for response. We conclude that gossypol is safe but unlikely to be clinically useful in patients with advanced cancer.