Background: Measurement of changes in serum antibodies is an excellent predictor of Helicobacter pylori eradication after antibiotic treatment. Aim: To measure the changes in serum antibody titers to Helicobacter pylori, before and after treatment. Material and methods: IgG antibodies to H. pylori were prospectively evaluated in 107 duodenal ulcer patients treated either with antibiotics (amoxicillin, metronidazole and bismuth subsalicylate) plus omeprazole alone. IgG antibody levels were determined using an "in house" ELISA in sera from 49 eradicated patients that received quadruple therapy and 58 non-eradicated patients (12 in whom antibiotic therapy failed and 46 that received omeprazole alone). Endoscopy, urease test, microscopy, and culture of gastric biopsies confirmed H. pylori eradication. Results: Patients in whom H. pylori was eradicated, showed a maintained drop in serum antibody titers that ranged from 15%, 62%, 74% to 76% at 28 days, 4, 8 and 12 months respectively. Such reduction was not observed in patients treated with omeprazole. Patients, in whom quadruple therapy failed to eradicate H. pylori, showed a discrete and transient decrease in antibody titers. By the fourth month, patients in whom eradication with quadruple therapy was not achieved, irrespective of whether they received quadruple therapy or omeprazole alone. Conclusions: A 45% decrease in IgG titer after 4 months is indicative of therapeutic success in H. pylori eradication. Therefore, serology may be useful to monitor the outcome of antibiotic therapy (Rev Med Chile 2000; 128: 1119-26).