We suggest "new" response criteria for phase II studies of supratentorial malignant glioma and favor rigorous criteria similar to those in medical oncology, with important modifications. Four response categories are proposed: complete response (CR), partial response (PR), stable disease (SD), and progressive disease (PD). Response in this scheme is based on major changes in tumor size on the enhanced computed tomographic (CT) or magnetic resonance imaging (MRI) scan. Scan changes are interpreted in light of steroid use and neurologic findings. We advocate careful patient selection, emphasize pitfalls in the assessment of response, and suggest guidelines to minimize misinterpretations of response.