Sixty-seven consecutive patients infected with the human immunodeficiency virus (HIV-1), 72% of which with overt AIDS, were examinated by upper endoscopy due to various indications and evaluated for the prevalence of H. pylori infection. The infection was studied by performing both histological examination of gastric biopsies and serological testing for anti-H. pylori IgG antibodies. The H. pylori prevalence rate was 55% in histology; no significant differences were observed in HIV-infected subjects and those with overt AIDS (52% vs 63%, respectively; P = NS). Positive histological testing appeared to be directly related to the peripheral CD4+ lymphocyte count (minimum rates of 43% were detected in patients with CD4+ < 100 x 10(6)/liter and maximum rates of 78% in patients with CD4+ > 200 x 10(6)/liter, respectively; P < 0.05) and inversely related to the frequency of antibiotic treatments performed over the six months prior to endoscopy. Low CD4+ counts were also apparently associated with low-grade H. pylori infection. Serological testing was positive for anti-H. pylori IgG antibodies in 39% of patients; compared to histology, serology displayed a sensitivity of 57% and a specificity of 81%. The discrepancy between histological and serological positive results for H. pylori was noted to be higher in the more advanced phases of HIV infection. Based upon our results, the serological testing for anti-H. pylori IgG antibodies seems to require cautious interpretation in HIV-positive patients.