Asthma and COPD are among the most common chronic non-communicable diseases. Many different environmental and individual factors are involved in their development. The main goals of treatment are to prevent symptoms of the disease, reduce the risk of exacerbations, and improve the patient's health. A common group of drugs used in the control of asthma and COPD are bronchodilator drugs. An example is formoterol, which as a long-acting selective β2-adrenoceptor antagonist, has a bronchodilating effect in patients with reversible airway obstruction. The aim of the study was to verify the correctness of operation of the commercially available DPI inhalers in the field of dry powder aerosolization containing formoterol fumarate. Comparative analyses of the two inhalers were performed using a Next Generation Impactor (NGI). For this purpose, formoterol fumarate in the form of inhalation powder in gelatin capsules was used. The quantification of the drug deposited on each impactor stage was determined by high performance liquid chromatography. The particle size distribution at various flow rates was determined and the extent of drug delivery to the lungs was estimated indirectly by determining the percentage of FPF showing the greatest therapeutic effect during inhalation. On the basis of the presented research results, it can be suggested that there is great difference in aerolization effect depending the utilized inhaler. The main reasons for the differences in the performance of the tested inhalers may be differences in the structure and properties of these medical devices. Although all capsule inhalers may look similar to the patient, even the smallest differences in their structure may affect aerosolization and the method of drug delivery to the lungs. Therefore, it is crucial, that the inhaler should be selected individually for each patient, taking into account the patient's age and inhalation possibilities. The studies were financed by a grant from the National Centre of Research and Development, Poland, LIDER Program, No: LIDER/52/0276/L-12/20/NCBR/2021.