1. The accuracy of an indirect immunofluorescence (IIF) test for the serodiagnosis of Helicobacter pylori infection was evaluated in adult patients and compared with culture, preformed urease test and smears stained with carbolfuchsin. 2. We studied 80 consecutive patients submitted to gastroduodenoscopy (17 of whom were found to have duodenal ulcer) plus 57 patients with duodenal ulcer. 3. H. pylori was identified by microbiological methods in 65 of the 80 consecutive patients and in all 57 patients with duodenal ulcer. 4. Among the 74 patients with duodenal ulcer, 71 presented antibody titers > or = 1:20 and 46 of the 48 H. pylori-positive patients without duodenal ulcer presented antibody titers > or = 1:20. 5. Thirteen of the 15 H. pylori-negative patients presented antibody titers < or = 1:10. 6. The sensitivity, specificity and positive predictive value of the IIF test were 95.9%, 88.8% and 98.4%, respectively. 7. The seroprevalence of H. pylori in 380 asymptomatic Brazilian blood donors was also studied by the IIF test. The presence of IgG antibodies against H. pylori was observed in 62.1% of the individuals. The prevalence of H. pylori infection increased with age and no difference was observed between males (60.3%) and females (66.6%).