BACKGROUND: Chronic obstructive pulmonary disease (COPD) patients may experience an acute exacerbation (AECOPD) that requires hospitalisation. The length of hospital stay (LHS) has a great economic impact on the health-care system. Knowing the predictors of prolonged LHS could help to identify possible interventions. METHODS: We performed a prospective study to identify the clinical predictors of prolonged LHS in patients hospitalised for AECOPD. We divided the study sample by LHS into normal (≤7 days) and prolonged LHS (> 7 days) groups. Outcomes were the need for non-invasive and invasive mechanical ventilation (NIMV and IMV), intensive care unit (ICU) admission, and the 3-year mortality. RESULTS: We enrolled 437 patients, of which 213 and 224 had normal LHS and prolonged LHS, respectively. Patients with a prolonged LHS had more prior hospitalisations for AECOPD, a worse mMRC (modified Medical Research Council) dyspnoea score, a higher prevalence of long-term oxygen therapy and a higher rate of congestive heart disease. During the current admission, this group also tended to require NIMV, IMV and ICU admission and the mortality risks at 6 months, 1 year and 3 years were higher. In the multivariate regression analysis, an mMRC dyspnoea score ≥ 2 (odds ratio-OR 2.24; 95% confidence interval-CI 1.34 to 3.74; p = 0.002) and the presence of acute respiratory acidosis (OR 2.75; 95% CI 1.49 to 5.05; p = 0.001) predicted a prolonged LHS at admission. CONCLUSIONS: The presence of an mMRC ≥2 and acute respiratory acidosis at admission independently increased the risk of a prolonged LHS for AECOPD.