BACKGROUND: Physician-reported performance status (PS) is an important prognostic factor and frequently influences treatment decisions. To the authors' knowledge, the extent, prognostic importance, and predictors of disagreements in PS assessment between physicians and patients have not been adequately examined. METHODS: Using North Central Cancer Treatment Group (NCCTG) clinical trial data from 1987 through 1990, the authors compared PS (Eastern Cooperative Oncology Group [ECOG] and Karnofsky [KPS]) and nutrition scores reported by physicians and patients individually. Differences were analyzed using a Student t test for paired data and degree of disagreement by kappa statistic. The effect of disagreement on overall survival was determined by the Kaplan-Meier method and Cox regression analysis. Predictors of disagreement were identified by logistic regression. RESULTS: In all, 1636 patients with advanced lung and colorectal cancer had a median survival of 9.8 months (95% confidence interval [95% CI], 9.4-10.4 months). Percent disagreement between patients and physicians regarding KPS, ECOG PS, and nutrition score were 67.1%, 56.6%, and 58.0%, respectively. Physicians were more likely to rate patients better than individual patients were to rate themselves: ECOG (mean 0.91 vs 1.30; P < .0001), KPS (mean 83.3 vs 81.7; P < .0001), and nutrition score (mean 1.6 vs 2.1; P < .0001). Disagreement between patients and their physicians was associated with increased risk of death: KPS (hazards ratio [HR] of 1.16; 95% CI, 1.04-1.30 [P = .008]) and nutrition scores (HR of 1.44; 95% CI, 1.29-1.61 [P < .0001]) after adjustment for covariates. Patient sociodemographic factors that predict disagreement were identified. CONCLUSIONS: Physicians and patients frequently disagree regarding PS and nutritional status. Disagreement is associated with an increased risk of death in patients with advanced malignancies. These findings illustrate the limitations of physician-only assessed PS.