The best known members of genus Mycobacterium belong to M. tuberculosis complex. Other mycobacteria are known as nontuberculous mycobacteria (NTM). NTM less commonly cause a disease (mycobacteriosis), more often colonising respiratory tract. The presence of NTM is more common in immunocompromised patients and in those with a previous lung disease. The decrease in the incidence of tuberculosis is followed by increased incidence of NTM. Since tuberculosis has been declining in Croatia over the last 50 years, increasing incidence of NTM is expected. Growing incidence of chronic obstructive pulmonary disease (COPB) is contributing to this increase. NTM are ubiquitous and inhaling of aerosol particles constitutes the dominant route of infection. They are not transmitted via interhuman contact. In addition to pulmonary and skin infections, disseminated infections are also described. The treatment of mycobacteriosis is difficult and long. Besides using antituberculotic drugs such as rifampin and ethambutol, the therapies use fluoroquinolones; the introduction of macrolides has significantly improved the outcome of treatment.