Mycobacterial infections were confirmed in 629 HIV-infected persons (1.4%). Five patients had a generalized infection with Mycobacterium avium complex (MAC), four an extrapulmonary tuberculosis caused by Mycobacterium tuberculosis (M. tbc). In general, the tuberculosis was the first severe opportunistic infection, while infections with MAC were more frequent in patients with already manifest AIDS. Common to all patients were a septic temperature and definite shift to the left of neutrophil granulocytes. Four of five patients with MAC also had diarrhoea, and three of four with tuberculosis additionally had peripheral lymphomas. The chest x-ray films were normal in six of the nine patients. The diagnosis was made in six patients primarily by the microscopic demonstration of acid-fast bacteria in lymph node tissue or stool, in three patients by culture from blood or liver tissue. Microscopic stool examination was helpful: in three of five patients with MAC and one of two with M. tbc in the stool culture, acid-fast bacteria had already been demonstrated. In an individual case MAC infections could not be distinguished either clinically or morphologically from infections with M. tbc, but only by culture.