Phase II trials of ifosfamide have been performed with standard doses of 5 to 8 g/m2/course. In this phase I study, 29 patients were treated with a 4-day continuous infusion ifosfamide to determine the maximum-tolerated dose and the nonhematologic dose-limiting toxicity. Autologous bone marrow support was to have been used for the subsequent dose level if granulocytes were more than 500/microL for more than 14 days in two of two to five patients at a given dose level. Doses were escalated from 8 to 18 g/m2 ifosfamide. Mesna was given at an equivalent dose by continuous infusion for 5 days. At the 18 g/m2 dose level, dose-limiting renal insufficiency and a median of 11 days (range, 8 to 18 days) of granulocytopenia (less than 500/microL) were observed. Thus, autologous bone marrow reinfusion ws not used. The duration of myelosuppression, the frequency and severity of mucositis, and renal tubular acidosis were all dose-dependent. Mild to moderate CNS toxicity also appeared to be related to dose; however, severe CNS toxicity (transient confusion, hallucinations, and somnolence) was observed sporadically at both low- and high-dose levels. Transient hematuria (greater than 50 red blood cells [RBCs]/high power field) occurred once but did not affect treatment. There were nine responses (two complete) in 27 heavily pretreated assessable patients including seven responses in 20 patients with advanced refractory sarcoma. Ifosfamide with mesna uroprotection can undergo considerable dose escalation over the usual prescribed doses before nonhematologic dose-limiting toxicity is encountered. Ifosfamide has broad cytotoxicity against solid tumors and may prove to be an important addition to high-dose combination chemotherapy regimens.