What's known on the subject? and What does the study add? Inhaled anticholinergic drugs have been associated with the risk of acute urinary retention (AUR), but this association was never studied under real life circumstances nor was this risk ever quantified. Use of inhaled anticholinergic drugs increases the risk of AUR by 40%. The risk of AUR is highest in recent starters, in patients with benign prostatic hyperplasia (BPH), and in patients receiving their anticholinergic drugs via nebulizer. It might be advisable to consider alternatives for inhaled anticholinergic drugs in COPD patients with BPH. OBJECTIVE To investigate the association between the use of inhaled anticholinergic drugs and the risk of acute urinary retention (AUR) under real-life circumstances. PATIENTS AND METHODS We conducted a nested case-control study within a cohort of patients with chronic obstructive pulmonary disease (COPD; as AUR has been associated with the use of inhaled anticholinergic drugs, which are used as first-line treatment for COPD) from the Integrated Primary Care Information (IPCI) database. The cohort consisted of all patients with COPD aged >= 45 years, registered between 1996 and 2006, with >= 12 months of valid history. Cases were patients with a first diagnosis of AUR. To each case, controls were selected matched for age, gender and index date. Multivariate conditional logistic regression analysis was used to calculate adjusted odds ratios (OR(adj)) with 95% confidence intervals (95% CI). RESULTS Within the cohort of 22 579 patients with COPD, 209 cases were identified. Current use of inhaled anticholinergic drugs was associated with a 40% increase in risk for AUR (OR(adj) 1.40; 95% CI 0.99-1.98) compared with non-users. Among current users, the risk was highest for the recent starters (OR(adj) 3.11; 95% CI 1.21-7.98). The risk of long-acting anticholinergic drug tiotropium was not substantially different from that of the short-acting anticholinergic ipratropium. The association was not dose-dependent, but changed by mode of administration, with nebulizers having the highest risk (OR(adj) 2.92; 95% CI 1.17-7.31). In men with COPD and benign prostatic hyperplasia (BPH) the association was strongest (OR(adj) 4.67; 95% CI 1.56-14.0). CONCLUSION Current use of inhaled anticholinergic drugs increases the risk of AUR, especially in patients with BPH or if administered via a nebulizer.