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BACKGROUND: Understanding the prognostic meaning of early COPD stages in the general population is relevant for discussions about under-diagnosis. So far, COPD prevalence and incidence were often estimated using pre- instead of post-bronchodilation spirometry. In the SAPALDIA Cohort we investigated time course, clinical relevance and determinants of severity stages of obstruction using pre-bronchodilator spirometry. METHODS: Incident obstruction was defined as FEV1/FVC ratio >/=0.70 at baseline and <0.70 at follow-up, non-persistence inversely. Determinants were assessed in 5490 adults with spirometry and respiratory symptom data in 1991 and 2002 using Poisson regression controlling for self-declared asthma and wheezing. Change in obstruction severity (defined analogously to GOLD classification) over 11 years was related to shortness of breath and health service utilization for respiratory problems by logistic models. RESULTS: Incidence rate of obstruction was 14.2 cases/1'000 person years. 20.9% of obstructive cases (n=113/540) were non-persistent. Age, smoking, chronic bronchitis and non-current asthma were determinants of incidence. After adjustment for asthma, only progressive stage I or persistent stage II obstruction was associated with shortness of breath (OR 1.71 (0.83-3.54), OR 3.11 (1.50-6.42) respectively) and health service utilization for respiratory problems (OR 2.49 (1.02-6.10), OR 4.17 (1.91-9.13) respectively) at follow-up. CONCLUSIONS: The observed non-persistence of obstruction suggests that pre-bronchodilation spirometry, as used in epidemiological studies, might misclassify COPD. Future epidemiological studies should consider both pre- and post-bronchodilation measurements and take specific clinical factors related to asthma and COPD into consideration for estimation of disease burden and prediction of health outcomes, respectively.