To determine the relationship between levels of Mycobacterium avium complex (MAC) in blood and tissues, histopathologic examination and quantitative MAC cultures were done on blood samples and tissue samples of 7 organs at autopsy of 10 AIDS patients who had been treated for MAC bacteremia. Blood and tissue cultures were negative for MAC for 3 of the patients and positive for 7. The numbers of MAC colony-forming units in the blood and tissues were highly correlated. The highest concentrations of MAC were observed in the reticuloendothelial organs, with a maximum of 6.9 log10 cfu/g in mesenteric lymph nodes and 6.8 log10 cfu/g in spleen. Histopathologic findings paralleled quantitative cultures and were consistent with entry of MAC via lymphatics through the gastrointestinal tract, followed by hematogenous dissemination.