This phase I study was designed to develop a high-dose combination of two cycles of mitoxantrone and cyclophosphamide in patients with solid tumors, as an alternative to single-cycle high-dose regimens that use only alkylating agents. Treatment was delivered with granulocyte colony-stimulating factor (G-CSF), but without stem cell support, in order to avoid potential tumor cell reinfusion. Thirty-one patients with advanced solid tumors received two cycles of high-dose mitoxantrone (20-30 mg/m2) plus high-dose cyclophosphamide (3000-4000 mg/m2). All patients received G-CSF until hematologic recovery. Dose-escalation was performed when less than 50% of cycles per level had dose-limiting toxicity (DLT). The maximum tolerated dose (MTD) achieved was mitoxantrone 25 mg/m2 and cyclophosphamide 4000 mg/m2. Main dose-limiting toxicities (DLTs) were hematological: grade IV neutropenia lasting more than 7 days and thrombopenia below 20 x 10(9)/l requiring more than one platelet transfusion. Non-hematological DLT consisted predominantly of grade III emesis and asthenia. Follow-up after each cycle was performed in an outpatient setting and there were no toxic deaths. In conclusion, the administration of two cycles of high-dose mitoxantrone and cyclophosphamide with G-CSF support is safe and feasible. MTD was mitoxantrone 25 mg/m2 and cyclophosphamide 4000 mg/m2. Evaluation of this regimen is being done in a phase II trial.