As all of the guidelines on the management of H. pylori infection suggest, the assessment of the eradication is generally performed at least 4 weeks after the completion of eradication treatment. However, H. pylori occasionally re-appears after one month, even the successful eradication was confirmed by the guideline. In this study, we investigated the appropriate time for the assessment of H. pylori eradication, mainly by using 13C urea breath tests (UBT) as a positive standard. From July 1992 to December 1997, 386 patients with H. pylori infection received eradication treatments. The presence of H. pylori was assessed by rapid urease test, UBT, culture and histologic examination. Eradication of the bacteria was determined by the negative results in all of these four tests. At 4 weeks after completion of therapy, 312 cases (80.8%) were judged as being free of H. pylori. Mean observation period was up to 12 months, and 113 cases were followed up to more than 1 year, and 50 cases were followed up to more than 2 years. H. pylori had re-appeared in 3 cases after 3 months, 1 case after 6 months, 2 cases after 12 months, and 1 case 24 months after the treatment, respectively. For the purpose of more accurate diagnosis, the assessment of eradication of H. pylori should be performed at 1 year after the completion of therapy. Since all the recrudescence could be diagnosed with UBT earlier and be confirmed by the other tests later, UBT is recommended as a useful methods in the assessment of Helicobacter pylori eradication.