Summary Nowadays the morbidity rate of nontuberculous mycobacteriosis is rising worldwide. The growing number of patients with microbiologically confirmed diagnosis of nontuberculous mycobacteriosis causes the increasing interest in the problem. The diagnostic of this illness is a real challenge due to the absence of a typical disease pattern. Objective. A case study will help to demonstrate pulmonary mycobacteriosis caused by Mycobacterium chelonae (M. chelonae) and to show the difficulties in the differential diagnosis between fibrocavitary tuberculosis and cavitary nontuberculous mycobacterial lung disease. Results. The clinical observation of a 40-year-old female, who has been long-term treated against fibrocavitary tuberculosis of the left upper lobe, is made. Subsequently M. chelonae lung disease was verified. Conclusion. As a result of this clinical case’s analysis, we can say that one of the main difficulties in diagnosing nontuberculous mycobacteriosis (incl. caused by M. chelonae) is the clinical and radiological similarities with pulmonary tuberculosis. Further errors in the diagnosis are caused by the complicated detection and identification of nontuberculous mycobacterias (species).