Résumé : Commercially available complement fixation test reagents (Institute Virion Ltd., Ruschlikon, Zurich, Switzerland) available in package format were evaluated for the serodiagnosis of Helicobacter pylori infection. The assay was compared with bacterial culture and histological Giemsa stain of gastric biopsy specimens obtained from 930 patients of different ages and from different ethnic groups, with a variety of upper gastrointestinal tract symptoms. The prevalence, sensitivity, specificity, and positive and negative predictive values, respectively, were 35, 71, 90, 80, and 85% for Belgian patients aged 40 years or younger, 50, 81, 93, 92, and 83% for Belgian patients older than 40 years, and 83, 83, 79, 95, and 48% for Mediterranean patients. Using 645 serum specimens from 226 patients, we also evaluated the complement fixation test for its ability to monitor the eradication of H. pylori following antimicrobial therapy. Overall, H. pylori was eradicated from 122 patients while 104 patients remained infected with the organism. A significant decrease in antibody levels was observed 3 to 6 months after the end of therapy in the group of patients from whom H. pylori was eradicated.