The clinical, microbiologic, and immunologic parameters in HIV-infected subjects first presenting with disseminated Mycobacterium avium complex (DMAC) were determined. Four HIV-positive groups not yet on DMAC treatment were enrolled: 19 subjects with CD4 lymphocyte counts < or =50/microl thought to have DMAC on clinical grounds; 18 subjects newly found to have a positive blood culture for MAC; 25 asymptomatic controls (CD4 cell counts < or =50); and 25 asymptomatic controls (CD4 counts 100-250/microl). Outcome measures include comparisons between groups for clinical characteristics; results of cultures from blood, marrow, and gastrointestinal and respiratory tracts; immunological markers from staining of marrow and flow cytometry of circulating lymphocytes; and cytokine production of PBMCs. Only 21% of the 19 patients entered on suspicion of having DMAC grew MAC from blood or marrow. Neither clinical presentation nor laboratory tests differentiated those culture-positive from those culture-negative patients. However, prior PCP or multiple other opportunistic infections were more common in the DMAC group. MAC was isolated from 82% of marrow and 50% of blood specimens from the DMAC group. Respiratory or gastrointestinal colonization was present in 36% of DMAC subjects, but only 5% of non-DMAC subjects with CD4 counts <50 cells/microl. CD8+ cells were more frequent in bone marrow, and CD4 cells recognizing MAC antigen were more frequent in blood from DMAC subjects vs. controls. Results suggest an early stage of tissue dissemination preceding persistent bacteremia, and mucosal entry without persistence of colonization. MAC-specific T cell responses apparently develop and persist during DMAC, but are dysfunctional or too infrequent to prevent persistence.