Our aim was to determine concordance between 13C-urea breath test and serology in detecting Helicobacter pylori and to study their potential use for monitoring eradication in patients with gastric ulcer. We prospectively studied 73 gastric ulcer patients. On endoscopy, biopsies were taken for hematoxylineosin staining and rapid urease testing. Blood samples were drawn for immunoglobulin G antibody determination by enzyme-linked immunosorbent assay (ELISA). A 13C-urea breath test was performed as well. Histology, serology, and urea breath tests were all repeated 1, 6, and 12 months after therapy completion in 56 infected patients. A proportion of positive agreement between serology and breath test results as high as 0.95 was found. McNemar statistic was 3 (p = 0.08), whereas kappa statistic was 0.83 (p < 0.0001). At month 6, significant differences in patients successfully treated relative to baseline serologic values were observed (chi2 = 11.7; p < 0.001). The area under the receiver operating characteristic (ROC) curve for diagnostic efficiency was 0.76, sensitivity was 74%, and specificity was 90% (for H. pylori eradication) when the fall of at least one category in serologic levels was considered as cut-off point. No further decreases in serologic levels were noted over the next 6 months, and 48.8% of patients remained seropositive 1 year after completion of successful treatment. A high concordance between serology and 13C-urea breath test results is observed when the two procedures are used for H. pylori infection diagnosis in patients with gastric ulcer. Also, serology can be successfully used for monitoring H. pylori eradication 6 months after therapy completion.