OBJECTIVE The study was designed to assess the toxicity and activity of high-dose ifosfamide (HDI) administered by continuous infusion at a dose of 4 g/m2/d over 3 days every 4 weeks in adult patients with advanced soft tissue sarcomas (ASTS) pretreated with doxorubicin and/or a standard-dose ifosfamide (SDI)-containing regimen. METHODS Between January 1991 and November 1993, 40 patients with progressive ASTS were entered onto the study. Twenty-eight patients had been pretreated with a multidrug regimen that contained SDI and were classified as follows: SDI-refractory (n = 21), SDI-resistant (n = 2), and indeterminate SDI-sensitive (n = 5). Patients were treated until progression or major toxicity. RESULTS One hundred forty-seven cycles of HDI were administered. Neutropenia was dose-limiting, with 100% of patients experiencing grade 3 to 4 toxicity and 12 admissions for febrile neutropenia (30% of patients). Neurotoxicity (17% of patients) was significantly associated with acute renal failure (n = 4) (P < .001), grade 4 thrombocytopenia (P < .01) and febrile neutropenia (P = .048). Chronic renal toxicity (n = 4) was significantly associated with retroperitoneal masses and/or prior nephrectomy (P = .008). Partial responses (PRs) were observed in 12 of 36 assessable patients (33%) and eight patients (22%) experienced disease stabilization. All but one response occurred in patients pretreated with SDI, with five PRs among SDI-refractory patients. Leiomyosarcomas appear resistant to HDI. The median response duration was 8 months (range, 6 to 13+) and the median overall survival time was 12 months. CONCLUSIONS The activity of HDI in these pretreated ASTS patients and the apparent circumvention of SDI resistance suggest a real dose-response relationship for ifosfamide and deserve further evaluation. Although toxic, this treatment appears feasible and manageable using routine clinical support. Since prophylaxis of ifosfamide-induced renal damage remains unknown, frequent monitoring of renal and tubular functions during therapy is highly recommended.