The objective of the present study was to ascertain the clinical features, risk factors, microbiologic spectrum and course of disease after treatment of infections by environmental mycobacteria (EM) in patients with and without HIV infection in our community. Eighty-eight patients with diseases caused by EM diagnosed between 1989 and 1997 were studied; 46 (52.7%) were HIV-positive. Mycobacterium kansasii was the most prevalent pathogen (54%) overall, followed by M. avium complex (40%). However, M. avium complex was most prevalent among HIV-positive patients (61%) and M. kansasii was most prevalent among HIV-negative patients (76%). Localized lung infections were most common among HIV-negative patients, whereas 74% of HIV-positive patients had disseminated disease. Among HIV-negative patients, chronic obstructive pulmonary disease and corticosteroid use were common associations. Pulmonary disease was subacute and non-specific in both patient groups, whereas abdominal pain was the first symptom of most patients with disseminated disease. On the chest films of 76% of the HIV-negative patients, we observed cavitation and infiltrates; 60% of HIV-negative patients had normal x-rays. No differences in antibiotic sensitivity were observed between strains from HIV-positive and HIV-negative patients. The prognosis was good in the HIV-negative group with combined therapy with 2 to 4 first-line antituberculous drugs, whereas response was poor in HIV-positive patients in spite of prolonged treatment with 3 to 5 drugs. Nevertheless, thanks to the highly effective anti-retroviral treatment of recent years, we seem to be observing improved response to therapy with less aggressive forms of EM disease.