We have evaluated the activity of ifosfamide in 75 patients with recurrent sarcomas and pediatric solid tumors. All patients had previously received cyclophosphamide in combination with other chemotherapeutic agents. Ifosfamide was administered as a continuous 5 day infusion at a dose of 1800 mg per M2, except in the last 14 patients who received the drug as a daily one hour infusion at the same dose level. Partial response was observed in 9 of 20 patients with Ewing's sarcoma, 2 of 9 patients with rhabdomyosarcoma, 3 of 17 patients with osteogenic sarcoma and 4 of 29 patients with various other neoplasms. A further 6 patients had stable disease, defined as the absence of progression for at least 6 cycles of therapy. Thus overall response rate was 24%, with the highest response rate of 45% being observed in Ewing's sarcoma. Toxicity was acceptable, although there was quite marked leucopenia (median nadir 700) with less profound thrombocytopenia (median nadir 87,000). Sepsis occurred in 3 patients but no patient died as a result of infection. Hematuria occurred in 43% of patients who did not receive mesna, and in 26% of patients who did, although prior pelvic irradiation was found to be a significant risk factor for hematuria. Only 1 of 14 patients without prior pelvic irradiation or hematuria developed hemorrhagic cystitis when treated with ifosfamide and mesna. Confusional states developed in 6 patients. We conclude that ifosfamide is an active agent in patients with relapsed sarcomas and childhood solid tumors, even when such patients have been previously treated with cyclophosphamide.