At present the increased incidence of infections with mycobacteria other than tuberculosis and leprae bacilli seems to be correlated with several causes: improved diagnostic techniques, prolonged life expectancy, immunodepression. Rational chemotherapy depends upon the identification of the etiologic mycobacterium and the determination of its drug susceptibility. Besides the "classic" treatment with 3 or 4 antituberculous and, sometimes, nonantituberculous chemotherapics, clinical trials are in progress to assess the effectiveness of new molecules: rifamycin derivatives, fluorinated quinolones, anti-lepromatous drugs, and the latest macrolides. Yet at present, national and international data do not permit to define a standard treatment for every mycobacteriosis; in fact, the drug resistance is high and varies not only between different strains but also within the same strain; moreover, there are discrepancies between in vitro and in vivo results. When possible, appropriated surgery for circumscribed disease is recommended.