Summary: Respiratory tract infections, especially lower respiratory tract infections, are a leading cause of death and disability worldwide. Causes of lower respiratory tract infections can be bacteria, viruses and fungi. Recurrent pneumonia is defined as two or more episodes of pneumonia at least one month apart over a 3-year period. This article aims to summarize the most common causes of recurrent lower respiratory tract infections in adults and discuss their differential diagnosis in the hospital setting. Contributing causes may include COPD, the presence of lung cancer, swallowing disorders and retractile pneumonia, tuberculosis, chest deformities, infective endocarditis, pulmonary embolism with pulmonary infarction, systemic vasculitis, and primary and secondary immunodeficiencies. Differential diagnosis is based on additional investigations, such as chest CT scan, bronchoscopy with collection of material for microbiological and histopathological examination, biochemical, morphological, serological tests and determination of immunoglobulin concentrations. In uncertain cases, a surgical lung biopsy may be performed. Recurrent lower respiratory tract infections are a significant clinical problem that can be a cause of death and a significant reduction in patients' quality of life. In view of these facts, it requires an in-depth diagnosis, including imaging, endoscopic and microbiological studies.