Unreported exacerbations and failure to seek medical attention may have consequences on the health of patients with chronic obstructive pulmonary disease. 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. The study is based on a multicenter Canadian cohort of patients with chronic obstructive pulmonary disease. 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 2 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. The study enrolled 421 patients. The overall incidence of diary exacerbations was 2.7 per person per year, but only 0.8 per person per year was reported. Predictors of reporting included age (HR [hazard ratio], 0.90; 95% confidence interval [CI], 0.81-0.98 per 5-yr increase), FEV(1)% 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 vs. weekday). There was a clinically important decline in health status for 52% of patients with reported exacerbation and 43% with unreported exacerbations. 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 an exacerbation, and both reported and unreported exacerbations had an impact on health status.