Objectives: This study aims to determine the incidence of reported and unreported exacerbations, to identify predictors of reporting, and to compare the impact of reported and unreported exacerbations on health status. Methods: The study is based on a multi-center Canadian cohort of COPD patients. Measurements: Patients completed a daily diary from which exacerbations were defined as a worsening of at least one key symptom (dyspnea, sputum amount, sputum color) recorded on at least two consecutive days. Patients were asked to contact the study center if there was a sustained worsening of symptom. Reported exacerbations were events that led to contacting study center or health care visit. Main Results: The study enrolled 421 patients. The overall incidence of exacerbations was 2.7 per person per year, and 1.9 per person per year were not reported. Predictors of reporting included age (HR 0.90; 95% CI 0.81-0.98 per 5 year increase), FEV1 % predicted (HR 0.84; 95% CI 0.70-0.99 per 10% increase), number of symptoms at onset (HR 1.59; 95% CI 1.37-1.84 per additional symptom) and time of the week (HR 0.35; 95% CI 0.22-0.56 weekend versus weekday) where HR is hazard ratio and CI is confidence interval. There was a clinically important decline in health status for 52% of patients with reported exacerbation and 43% with unreported. Conclusions: This study has shown that less than one third of the exacerbations were reported. The number of symptoms at onset was the most important predictor of reporting exacerbation, and both reported and unreported xacerbations had an impact on health status.