Mycobacterium avium complex (MAC) is a group of ubiquitous and opportunistic bacterial pathogens included on the U.S. Environmental Protection Agency Drinking Water Contaminant Candidate List. The risk of contracting a disseminated MAC infection is primarily limited to the immunocompromised, including those with advanced acquired immunodeficiency syndrome (AIDS). These infections likely result from exposures to MAC-contaminated tap water, food, or soil, although the epidemiologic evidence is insufficient to implicate a specific medium. The objective of this study was to assess tap water exposure to MAC in the U.S. population with advanced AIDS, defined here as having fewer than 100 CD4(+) cells/mm(3) of blood. Using limited data on the detection of MAC and self-reported post-tap treatment practices, two exposure models were developed to simulate the likelihood of exposure to MAC via tap water consumption in this sensitive population. The first model integrated data from studies that described sources of water for consumption and post-tap treatment rates in cohorts infected with human immunodeficiency virus (HIV(+)). The second model used data from a study that categorized the fraction of water intake consisting of tap water that was not further treated. Approximately 1500 individuals with advanced AIDS were estimated to ingest tap water with detectable concentrations of MAC organisms daily. Additional studies on tap-water use in U.S. HIV(+) populations are needed to confirm these findings. Longitudinal and cross-sectional studies on the occurrence of MAC in tap water, particularly in regions with large HIV(+)/AIDS populations, would help address some of the uncertainty in these exposure estimates.