Summary Proper inhaler technique is crucial for effective management of asthma and COPD. This multicentre, cross-sectional, observational study investigates the prevalence of inhaler mishandling in a large population of experienced patients referring to chest clinics; to analyze the variables associated with misuse and the relationship between inhaler handling and health-care resources use and disease control. We enrolled 1664 adult subjects (mean age 62 years) affected mostly by COPD (52%) and asthma (42%). Respectively, 843 and 1113 patients were using MDIs and DPIs at home; of the latter, the users of Aerolizer® , Diskus® , HandiHaler® and Turbuhaler® were 82, 467, 505 and 361. We have a total of 2288 records of inhaler technique. Critical mistakes were widely distributed among users of all the inhalers, ranging from 12% for MDIs, 35% for Diskus® and HandiHaler® and 44% for Turbuhaler® . Independently of the inhaler, we found the strongest association between inhaler misuse and older age ( p  = 0.008), lower schooling ( p  = 0.001) and lack of instruction received for inhaler technique by health caregivers ( p  < 0.001). Inhaler misuse was associated with increased risk of hospitalization ( p  = 0.001), emergency room visits ( p  < 0.001), courses of oral steroids ( p  < 0.001) and antimicrobials ( p  < 0.001) and poor disease control evaluated as an ACT score for the asthmatics ( p  < 0.0001) and the whole population ( p  < 0.0001). We conclude that inhaler mishandling continues to be common in experienced outpatients referring to chest clinics and associated with increased unscheduled health-care resource use and poor clinical control. Instruction by health caregivers is the only modifiable factor useful for reducing inhaler mishandling