Infection due to the Mycobacterium avium complex (MAC) accounts for the most frequent AIDS-related opportunistic infections, but MAC infection is usually not the first AIDS-defining event that a patient infected with HIV experiences. The incidence increases linearly over time, at a rate of 20 to 25% per year, after a patient's first AIDS-defining event, and the incidence increases exponentially as the CD4+ cell count approaches zero. There is evidence that MAC may eventually infect most if not all HIV-infected patients who do not die from another HIV-related event. Since MAC infection contributes substantially to the morbidity and mortality of AIDS patients, prophylaxis appears to be mandatory. Rifabutin was the first drug which was shown to be effective in preventing MAC infection, and, recently, prophylaxis with clarithromycin was also found to prevent the disease. The optimal approach to prophylaxis still needs to be defined. Since a large majority of MAC infections occur in patients with CD4+ cell counts below 50/microliter, recommendations regarding the prophylaxis of patients with a history of an AIDS-defining opportunistic event and a CD4+ cell count between 50 and 200/microliter can be individualized, depending for example on how well the patient seems to be responding to antiretroviral treatment. Prophylaxis against MAC should be provided for any HIV-infected patient with a CD4+ cell count less than 50/microliter.