OBJECTIVES: Estimation of health gains associated to early diagnosis and treatment of Chronic Obstructive Pulmonary Disease (COPD) in Portugal over a 20 year horizon.   METHODS: A stochastic individually based dynamic microsimulation model was built generating the evolution of diagnosed and undiagnosed COPD prevalences and severity distribution over four stages. Exacerbation frequencies and quality of life were associated to severity stages. Smoking habits were taken into account since smoking is the main risk-factor of COPD and smoking cessation is the most effective strategy to slowdown disease progression. The model defines annual probabilities for individuals to develop COPD according to sex, age and smoking habits. Individuals with COPD progress depending on disease stage and smoking behavior. Intervention health benefits were estimated as the difference between the status quo and an intervention scenario. In the intervention scenario, smokers and ex-smokers aged 40 and over are screened in primary healthcare settings and selectively referred to smoking cessation programs. When diagnosed with COPD, patients progress more slowly and have higher quality of life due to medication and to less frequent exacerbation episodes. Health benefits of the intervention were estimated as the population life-years (LY) and quality-adjusted life-years (QALYs) difference between the scenarios. RESULTS: Over the 20 years more than 2.8 million individuals are screened. With discounting at 5%, the intervention resulted in gains of 33,230 LY for men and 16,990 LY for women while incremental QALYs were 91,586 and 25,983 respectively. Serious exacerbation events are 2.4% lower in the intervention scenario.   CONCLUSIONS: These results show that early diagnosis and early treatment could lead to substantial health gains for the Portuguese population in a 20 year time horizon.