We have carried out a large prospective study of the frequency of H. pylori infection and HIV-1 status in a community of ex-drug abusers including subjects with (N = 210) and without (N = 259) upper gastrointestinal symptoms, endoscopy and serology. Control groups were patients with upper gastrointestinal symptoms not at high risk of HIV-1 infection (N = 219) and asymptomatic blood donors (n = 322). H. pylori was present in 52% of symptomatic community residents having endoscopy and 55% of the control patients with symptoms but not at high risk of HIV-1 infection. H. pylori was less common in HIV-1-positive patients (40%) than those who were negative (66%; P < 0.001). In patients with AIDS (33%), the frequency of H. pylori infection was reduced compared to HIV-1-positive patients without AIDS (53%; P = 0.05). All the residents with AIDS had upper gastrointestinal symptoms. In community residents, peptic ulcer was always associated with H. pylori infection. By H. pylori serology, there was no difference in the frequency of infection in asymptomatic residents (56%) whether HIV-1 positive (55%) or HIV-1 negative (58%) compared with those residents with symptoms. Overall, H. pylori was less common in HIV-1-positive residents (49%) than those who were HIV-1 negative (61%; p < 0.05). This difference was due mainly to the low frequency of infection in residents with AIDS (33%). H. pylori infection is common in HIV-1 positive patients, and only slightly reduced when compared with at-risk HIV-1-negative subjects. Peptic ulcer is associated with H. pylori in HIV-1 infection.(ABSTRACT TRUNCATED AT 250 WORDS)