The Karnofsky Performance Status appears to be the most widely used scale for objective assessment of medical patients' quality of life. Evidence for its reliability and validity is reviewed and new data presented. Tests of inter-rater reliability, concurrent validity, and discriminant validity indicate that, with standardized observational procedures based on a mental status exam, the Karnofsky scale is acceptably reliable and valid as a global measure but it does not adequately capture the conceptual domain of quality of life.