Objective: To evaluate knowledge of and techniques for using prescribed inhalation devices among patients with asthma or COPD treated at a tertiary teaching hospital. Methods: Patients were assessed after medical visits, and their physicians were blinded to this fact. Patients were asked to demonstrate their inhaler technique and were then interviewed regarding their knowledge of inhalation devices, control of the disease and instructions received during medical visits. Results: We included 120 volunteers: 60 with asthma and 60 with COPD. All of the asthma patients and 98.3% of the COPD patients claimed to know how to use inhaled medications. In the sample as a whole, 113 patients (94.2%) committed at least one error when using the inhalation device. Patients committed more errors when using metered-dose inhalers than when using the dry-powder inhalers Aerolizer® (p < 0.001) or Pulvinal® (p < 0.001), as well as committing more errors when using the Aerolizer® inhaler than when using the Pulvinal® inhaler (p < 0.05). Using the metered-dose, Pulvinal® and Aerolizer® inhalers, the COPD group patients committed more errors than did the asthma group patients (p = 0.0023, p = 0.0065 and p = 0.012, respectively). Conclusions: Although the majority of the patients claimed to know how to use inhalation devices, the fact that 94.2% committed at least one error shows that their technique was inappropriate and reveals a discrepancy between understanding and practice. Therefore, it is not sufficient to ask patients whether they know how to use inhalation devices. Practical measures should be taken in order to minimize errors and optimize treatment.Keywords: Metered dose inhalers; Nebulizers and vaporizers; Asthma; Pulmonary disease, chronic obstructive. ResumoObjetivo: Avaliar a compreensão e a técnica de uso dos dispositivos inalatórios prescritos de pacientes com asma ou DPOC atendidos em um hospital-escola terciário. Métodos: Os pacientes foram avaliados na sala de pós-consulta sem que o médico soubesse do estudo. Foi solicitado que o paciente demonstrasse como utiliza a medicação inalatória e, em seguida, esse foi entrevistado sobre a compreensão dos dispositivos inalatórios, controle da doença e orientações recebidas durante as consultas. Resultados: Foram avaliados 120 voluntários: 60 asmáticos e 60 com DPOC. Todos os asmáticos e 98,3% do grupo DPOC relataram saber utilizar os medicamentos inalatórios. Na amostra como um todo, 113 pacientes (94,2%) cometeram pelo menos um erro ao utilizar o dispositivo inalatório. Os pacientes cometeram mais erros ao utilizar aerossol dosimetrado do que ao utilizar os inaladores de pó seco Aerolizer® (p < 0,001) ou Pulvinal® (p < 0,001), assim como mais erros ao utilizar Aerolizer® do que ao utilizar Pulvinal® (p < 0,05). O grupo DPOC cometeu significativamente mais erros que o grupo asma ao utilizar o aerossol dosimetrado (p = 0,0023), Pulvinal® (p = 0,0065) e Aerolizer® (p = 0,012). Conclusões: Embora a maioria dos pacientes relatasse saber a técnica adequada de utilização do...