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BACKGROUND: Spirometry, the measurement of forced expiratory volume in 1 s and forced vital capacity, is recommended in the diagnosis and management of the obstructive lung diseases asthma and chronic obstructive pulmonary disease (COPD). The present report describes spirometry use in Manitoba and tests the hypothesis that regional spirometry use correlates with the prevalence of physician-diagnosed obstructive lung diseases. METHODS: Spirometry is renumerated on a fee-for-service basis by Manitoba Health. Like other physician services, billing data include a diagnosis, patient identifiers, as well as the patient's sex, date of birth and residential postal code. Physician billings for spirometry for 1991 to 1998 were analyzed, comparing data with billings for physician visits for obstructive diseases. Four age groups were examined, as were income quintiles in Winnipeg, Manitoba. In addition, the prevalence of physician-diagnosed obstructive diseases were compared with spirometry rates in 49 service use areas of the province. RESULTS: Annually, about 3% of the Manitoba population underwent spirometry, and in aggregate, about 14% underwent spirometry during the eight years of the study. Rates in Winnipeg were higher than in the remainder of the province. Spirometry rates did not increase with time, and people who underwent spirometry had 1.4 to 1.7 tests/year. In children, higher income quintiles were tested more than lower income quintiles, while in adults, income quintiles were tested with equal frequency. People with obstructive lung disease accounted for about 75% of those tested, and in people with these diagnoses, the likelihood of testing increased approximately linearly with the number of physician visits for asthma or COPD. Children with asthma were tested less often than adults, and adults with asthma or both asthma and COPD were tested more often than those with COPD alone. In adults with asthma or asthma and COPD who had more than 10 physician visits for these diagnoses, testing rates were more than 70%, and multiple tests were common. In patients labelled with COPD only and with more than 20 physician visits, about one-third did not undergo spirometry. In children aged five to 14 years and in adults 15 to 44 years old, regional spirometry rates correlated well with regional asthma rates. Regional spirometry rates also correlated significantly with regional rates of asthma and/or COPD in people older than 34 years old. INTERPRETATION: Spirometry use is considerably higher in patients with asthma than in patients with COPD, suggesting that guidelines are followed more closely in patients with asthma, and that many patients are labelled with COPD without appropriate documentation. Spirometry use is apparently indicative of physician interest in the problem of obstructive lung diseases.