Background: Not all the techniques for the correct use of metered dose inhalers are used by patients and health care professionals. Aim: To assess the techniques and errors in the use of metered dose inhalers among patients and health care professionals. Material and methods: Evaluation of the inhaling technique, using a validated questionnaire, used by 68 patients, 30 physicians and 30 nurses working in a hospital at Concepción, Chile. Results: The "closed mouth" technique is used by 84% of patients, 40% of physicians and 73% of nurses. The rest uses the "open mouth" technique. The aerochamber is used by 12% of patients, 37% of physicians and 27% of nurses. The most frequent inhaling errors detected were not shaking the inhaler (26% of patients, 30% of physicians and 7% of nurses), not applying the puff at the start of inspiration (28% of patients, 7% of physicians and 13% of nurses), and not maintaining an apnea after the inhalation (41% of patients, 7% of physicians and 10% of nurses). Sixty percent of patients, 67% of physicians and 40% of nurses have not received instructions about the inhaler use. Conclusions: There is a high frequency of errors in the use of inhalers and most users have not been trained in its use. (Rev Méd Chile 2001; 129: 413-20)