We aimed to describe changes in the prevalence of chronic obstructive pulmonary disease (COPD) in Spain by means of a repeated cross-sectional design comparing two population-based studies conducted 10 yrs apart. We compared participants from IBERPOC (Estudio epidemiológico de EPOC en España) (n = 4,030), conducted in 1997, with those of EPI-SCAN (Epidemiologic Study of COPD in Spain) (n = 3,802), conducted in 2007. Poorly reversible airflow obstruction compatible with COPD was defined according to the old European Respiratory Society definitions. COPD prevalence in the population between 40 to 69 yrs of age dropped from 9.1% (95% CI 8.1-10.2%) in 1997 to 4.5% (95% CI 2.4-6.6%), a 50.4% decline. The distribution of COPD prevalence by severity also changed from 38.3% mild, 39.7% moderate and 22.0% severe in 1997, to 85.6% mild, 13.0% moderate and 1.4% severe in 2007, and in the 40-69 yr EPI-SCAN sub-sample to 84.3% mild, 15.0% moderate and 0.7% severe. Overall, underdiagnosis was reduced from 78% to 73% (not a significant difference) and undertreatment from 81% to 54% (p<0.05) within this 10-yr frame. The finding of a substantial reduction in the prevalence of COPD in Spain is unexpected, as were the observed changes in the severity distribution, and highlights the difficulties in comparisons between repeated cross-sectional surveys of spirometry in the population.