Ifosfamide and mesna were administered to 77 patients with advanced malignancies. Seven (9%) experienced a severe but reversible encephalopathy. In 56% of patients in whom EEG data was available, characteristic changes were seen with or without mild clinical toxicity. Discriminant analysis identified low serum albumin concentration, high serum creatinine concentration and the presence of pelvic disease as variables which predispose patients to the development of severe encephalopathy. A nomogram has been constructed which can be used to determine the probability that an individual patient may be given ifosfamide and mesna safely. This has important implications for the clinical use of a highly active chemotherapy regimen.