Serial quantitative blood cultures were performed before and during treatment in four patients with the acquired immune deficiency syndrome (AIDS) and Mycobacterium avium-intracellulare bacteremia. Initial colony counts were 350 to 28,000 cfu/ml, the counts declined substantially with treatment in two patients, and they declined modestly with treatment but rose when it was stopped in the other two. In one patient who was studied in detail, most of the circulating organisms were within the leukocytes, colony counts in blood subjected to lytic agents were 1.9- to 5.2-fold higher than in unlysed blood, and there were 10(5) to 10(6) times more organisms per gram in several tissue specimens obtained at autopsy than per milliliter of blood. It is concluded that continuous high-grade bacteremia is common in patients with AIDS and severe M. avium-intracellulare infections and that serial quantitative blood cultures provide a potential means for studying treatment in these patients.