Background: Health authorities create and maintain administrative databases. Despite the potential advantages of these databases, the validity of the information they include must be considered. Objective: To examine the validity of diagnosing chronic obstructive pulmonary disease (COPD) from a large administrative database. Methods: Physician services and prescription claims data related to COPD and asthma were extracted from the Quebec universal medical insurance register (Régie de l'assurance-maladie du Québec; RAMQ) from the period of April 1, 1994 to March 31, 1999. Before obtaining the data, criteria for the validity of the COPD diagnosis in the database were formulated based on the epidemiology of COPD in the province. The extent to which the database satisfied these criteria are described within the present paper. Results: For patients aged 65 years or older, COPD was two times more prevalent in the RAMQ database than in the 1994/1995 National Population Health Survey. One in three patients with a RAMQ-diagnosis of COPD also had a RAMQ-diagnosis of asthma, and 47% of patients aged 65 years or older with a RAMQ-diagnosis of COPD did not fill any prescription for beta-2-agonists. In addition, 42% of patients with a RAMQ-diagnosis of COPD who never had a RAMQ-diagnosis of asthma appeared only once with that diagnosis in the database. Of all patients aged 65 years or older with a RAMQ-diagnosis of COPD, 37% and 23% met the operational definitions of 'possible COPD' and 'probable COPD', respectively. Conclusions: Most RAMQ-diagnoses of COPD lack validity; therefore, the validity of database diagnoses should be routinely ascertained before using administrative databases in clinical and health services research. © 2005 Pulsus Group Inc. All rights reserved.