Organisms of the <i>Mycobacterium avium</i> complex cause disseminated infections in 25 to 50 % of patients with AIDS. To assess the likelihood of exposure to M avium, we attempted to recover M. avium complex from environmental samples in geographical areas (Boston, Massachusetts; Hanover, New Hampshire; Helsinki, Finland; Nairobi, Kenya; and Kinsasha, Zaire) located near <i>M. avium</i> infected AIDS patients. Although <i>M. avium</i> was recovered from environmental samples at all sites, it was found more frequently in water supply systems in the United States and Finland (8/25, 32 %) compared to water supply samples from Africa (0/14, 0%). To determine if <i>M. avium</i> isolates recovered from the same geographical area as AIDS patients shared phenotypic and genetic characteristics with clinical AIDS <i>M. avium</i> isolates (recovered by collaborating laboratories), the ability to grow at 43°C, cadmium-and streptomycin-resistance, and the presence of plasmids were used as epidemiological markers. We found that environmental isolates in this study shared similar characteristics with the clinical AIDS <i>M. avium</i> isolates. Compared to developed countries, the prevalence of <i>M. avium</i> infections among AIDS patients in developing countries (i.e., Africa) is very low. To determine if <i>M. avium</i> was absent in the African environment, we attempted to recover the organisms from water and soil in Kampala, Uganda. <i>M. avium</i> was recovered from 43 % of environmental samples, and these isolates shared similar phenotypic and genetic characteristics with <i>M. avium</i> isolates from the United States. Cigararette smoking was identified as a possible risk factor for HIV infected individuals. M avium isolates were recovered from several brands of cigarettes, suggesting that cigarettes are a possible source of infection. / Master of Science