OBJECTIVES: Although respiratory symptoms are a defining feature of COPD, a standardized method for assessing their day-to-day severity suitable for testing treatment effects in clinical trials has not been established. This study examined the reliability and validity of the E-RS (EXACT-Respiratory Symptoms), a respiratory symptom score derived from the 11 respiratory items of the 14-item EXACT, a daily diary assessing exacerbations of COPD. METHODS: Content validity was assessed through analyses of qualitative data from 84 patients with COPD, including secondary analyses of data collected during EXACT development (n=63) and data from four new focus groups with patients without recent history of exacerbation (n=21). Quantitative properties were tested through secondary analyses of data from 188 stable COPD patients gathered over 7 days during the first EXACT validation study. RESULTS: Qualitative: Sample mean (SD) age=65(10), 44% male, mean FEV1=1.2(0.4) L. Patient descriptions of respiratory symptoms during stable disease were consistent with the E-RS content, wording, and structure.  Quantitative: Sample mean (SD) age=66 (10), 50% male, mean FEV1=1.2(0.5) L.  Factor analysis showed three subscales: RS-Breathlessness (5 items), RS-Cough & Sputum (3 items) and RS-Chest (3 items).  For RS-Total and subscale scores, respectively: Reliability, internal consistency (alpha) =0.88 0.86, 0.73, 0.80; test-retest (ICC) Day 1 to 7 (n=171) =0.73; 0.71; 0.69; 0.62. Validity: Correlations (Spearman’s) with St. George Respiratory Questionnaire – COPD (SGRQ-C) =0.75; 0.69, 0.58, 0.52, modified Medical Research Council dyspnea scale (mMRC) =0.33; 0.38, 0.24, 0.16, and rescue medication use =0.32; 0.34, 0.26, 0.17 (p≤0.05 to 0.0001). CONCLUSIONS: Results suggest the E-RS is a valid and reliable method for evaluating severity of respiratory symptoms in COPD; the daily diary structure permits assessment of day-to-day variability and severity over time. Further research is needed to evaluate performance over longer assessment periods and in response to treatment, and to refine score interpretation.