Objective To examine the association between exacerbation frequency and mortality following an acute exacerbation of chronic obstructive pulmonary disease (AECOPD).

Design Cohort study using medical databases.

Setting Northern Denmark.

Participants On 1 January 2005, we identified all patients with prevalent hospital-diagnosed chronic obstructive pulmonary disease (COPD) who had at least one AECOPD during 1 January 2005 to 31 December 2009. We followed patients from the first AECOPD during this period until death, emigration or 31 December 2009, whichever came first. We flagged all AECOPD events during follow-up and characterised each by the exacerbation frequency (0, 1, 2 or 3+) in the prior 12-month period.

Main outcomes and measures Using Cox regression, we computed 0–30-day and 31–365-day age-adjusted, sex-adjusted, and comorbidity-adjusted mortality rate ratios (MRRs) with 95% CIs entering exacerbation frequency as a time-varying exposure.

Results We identified 16 647 eligible patients with prevalent COPD, of whom 6664 (40%) developed an AECOPD and were thus included in the study cohort. The 0–30-day MRRs were 0.97 (95% CI 0.80 to 1.18), 0.90 (95% CI 0.70 to 1.15) and 1.03 (95% CI 0.81 to 1.32) among patients with AECOPD with 1, 2 and 3+ AECOPDs versus no AECOPD within the past 12 months, respectively. The corresponding MRRs were 1.47 (95% CI 1.30 to 1.66), 1.89 (95% CI 1.59 to 2.25) and 1.59 (95% CI 1.23 to 2.05) for days 31–365.

Conclusion Among patients with AECOPD, one or more exacerbations in the previous year were not associated with 30-day mortality but were associated with an increased 31–365-day mortality.