The relationship between Mycobacterium avium complex (MAC) bacteremia and proinflammatory cytokine and human immunodeficiency virus type 1 (HIV-1) RNA levels in AIDS was investigated. During a prospective study, blood samples were drawn monthly for mycobacterial cultures. Sera were available at baseline and onset of MAC bacteremia from 20 cases and at corresponding times from 19 controls. Mean interleukin-6 (IL-6) levels were 154% greater at the time of MAC bacteremia in cases than in controls. The IL-6 levels correlated with body temperature, serum tumor necrosis factor (TNF-alpha) levels, and alkaline phosphatase levels (P < or = .004 for each). Although TNF-alpha levels tended to rise more in MAC patients than in controls, the difference was not significant. However, among both cases and controls, serum TNF-alpha levels rose significantly from baseline to the time of last sample, irrespective of MAC infection (P = .015). Bacteremia was not associated with increased serum HIV-1 RNA levels. Thus, early MAC bacteremia is associated with increases in serum IL-6 levels, while TNF-alpha levels rise over time during advanced AIDS.