The prevalence of infection with Mycobacterium avium complex (MAC) has increased since the outbreak of the HIV pandemic. This complex comprises two organisms: M. avium (mostly) and M. intracellulare (rarely). The source of MAC infection is not known. The principal risk factors for disseminated MAC infection in a patient with HIV infection are a low CD4 count and a previous opportunistic infection. The symptoms of disseminated MAC infection resemble those of HIV wasting; a positive culture of normally sterile tissue confirms a MAC infection. There is reserve with regard to routine prophylaxis in HIV-infected persons because of the possible development of resistance, interaction with other drugs used in AIDS, toxicity and possible absorption disorders which might cause prophylaxis to fail. For the treatment of disseminated MAC infection, a combination of at least two medicaments (macrolides and ethambutol) is recommended.