Chronic obstructive pulmonary disease (COPD) is one of the commonest respiratory diseases. During the early stage of COPD, patients only have mild respiratory symptoms or signs which may lead to under-diagnosis of the disease. Patients may show poor response to treatment at later stages of the disease, associated with higher mortality and incidence of re-hospitalization and disability causing burden for both the families and the society. So far, there is no large-scale clinical trial on long-term intervention with tiotropium bromide (Spiriva) in patients with early stages of COPD (i.e. GOLD Stage I-II COPD or asymptomatic COPD). It would be of great significance for COPD prevention and treatment if the investigators could prove that tiotropium decreases the lung function decline and reverses disease progression in patients with early-stage COPD. The investigators objective is to evaluate the efficacy of long-term intervention with tiotropium in early stage (FEV1 ≥50% predicted) COPD (difference of trough FEV1, number of exacerbations, time to first exacerbation, quality of life, etc) and relevant pharmacoeconomic endpoints.