Classification of atypical mycobacteria, problems related to their identification, epidemiological, clinical, radiological and pathohistological presentation of mycobacteriosis are given. Up-to-date alternatives for treatment of these patients are also given. Finally, three cases treated at the VII Clinical Ward of the Institute of Pulmonary Diseases and TBC in the course of 1990 are presented. In two of them Mycobacterium avium was isolated. In all three cases the same strains were isolated and identified repeatedly.