The magnitude and time course of steroid-induced CT changes were analyzed in 11 patients with recurrent malignant glioma. CTs were obtained before and at regular intervals after starting dexamethasone (16 mg/d). Midline shift, ventricular compression, edema, enhancement intensity, and the size of the enhancing mass often improved with steroid treatment. Improvement occurred within 2 weeks in most instances. Changes in the volume of the enhancing tumor were assessed quantitatively in eight patients. In six, the mass was smaller after 2 weeks of steroid therapy, and in two the reduction approached 50%. Steroid-induced CT changes can mimic treatment responses. If steroids are necessary for symptom control, patients should be taking these medications for 2 weeks before a baseline CT is obtained and investigational treatment started.