A phase I clinical study of aziridinylbenzoquinone (AZQ) was conducted in 33 patients with various types of advanced solid tumors to evaluate its toxicity and efficacy. The initial dose of 0.5 mg/m2/day X 5 days repeated at 3-week intervals was progressively increased to a maximum dose of 12.0 mg/m2/day. Thrombocytopenia was the dose-limiting toxic effect; it was delayed, cumulative, and occurred more often in patients with extensive prior chemotherapy and radiotherapy. Anemia was common and severe at higher doses, while nausea and vomiting were observed only in some patients and usually were mild. Objective tumor regressions were observed in 3 of 17 patients who received biologically active doses of AZQ, i.e., 6 mg/m2/day or higher. Minor responses were seen in two of three patients with malignant melanoma and in a patient with adenocarcinoma of unknown primary. The recommended starting dose of AZQ for good-risk patients is 8.0 mg/m2/day X 5 days for phase II studies.