Mitoquidone is pentacyclic pyrroloquinone derivative developed for clinical evaluation as a potential anticancer agent. Mitoquidone demonstrated good activity in a range of experimental solid tumour models, but was weakly active against standard prescreens such as the P388 murine leukaemia. Bone marrow suppression or other significant toxicity was not observed in preclinical studies. In clinical trials patients were treated with Mitoquidone given as a 4-h infusion either once every 21 days, once a week, or as 5 daily doses repeated every 28 days. The major adverse events encountered included nausea and vomiting (in virtually all patients), dyspnoea, tumour-related pain, and thrombocytopenia in several patients with pretreatment bone-marrow impairment. Phase I studies were suspended without a maximum tolerated dose being reached because of formulation difficulties. There were no major responses, although stable disease was observed in a number of patients with gastrointestinal malignancies.