BACKGROUND The diagnosis of obstructive lung disease (OLD) based on clinical grounds is challenging. There have been no population-based COPD studies that collected pulmonary function data in Portugal, a country in transition between phases 2 and 3 of the smoking epidemic. OBJECTIVE To estimate the prevalence of obstructive pattern on spirometry in a representative sample of adults from Porto, Portugal. METHODS We conducted a health survey between 2001 and 2003, and 758 participants ≥ 40 years old had reliable spirometry. We used a structured questionnaire to collect demographic, clinical, social, and behavioral data. Obstructive pattern was defined as FEV(1)/FVC < 70%. Logistic regression was performed to quantify the association between socio-demographic and clinical factors and outcome. RESULTS The participants' mean ± SD age was 58.5 ± 11.5 years, and 62% were women. The prevalence of spirometric obstructive pattern was 10.7%, 95% CI 8.6-13.1%; 13.4% in men, and 9.1% in women (P = .08). The age-adjusted odds ratios for cumulative smoking exposure of less than and more than 20 pack years, in comparison with never smokers, were 3.49 (95% CI 1.02-11.92) and 3.91 (95% CI 1.29-11.89) among men, and 1.47 (95% CI 0.53-4.08) and 2.68 (95% CI 1.07-6.68) among women, respectively. Previously diagnosed OLD was reported by 30.9% (95% CI 21.1-42.1%) of the participants with spirometric obstructive pattern. Spirometry confirmed the OLD diagnosis in 20.5% (95% CI 13.7-28.7%) of subjects who self-reported OLD (kappa = 0.14, 95% CI 0.07-0.20). CONCLUSIONS The prevalence of spirometric obstructive pattern was high. Considering Portugal's position in the smoking epidemic, together with the aging of the population, we can expect an increase in the prevalence of OLD in older people and in women. Our results confirm the limited validity of self-reported OLD in epidemiological studies.