Background: Recent reports suggest that Helicobacter pylori infection can potentially increase the risk of colorectal cancer. The purpose of this study was to assess the association between H. pylori infection and the risk of colorectal adenoma and adenocarcinoma, and to evaluate any differences on the basis of sex. Methods: The subjects were 669 (40-to 80-year-old) patients who underwent both barium enema examination and total colonoscopy, and who were evaluated for H. pylori infection by 13C-urea breath test, urease test, or histological diagnosis of biopsied gastric specimens. There were 142 H. pylori-negative and 527-positive patients. The odds ratios (ORs) for H. pyloripositive patients with colorectal adenoma and adenocarcinoma, and for tumor patients with either adenoma or adenocarcinoma were calculated. Results: Among the H. pylori-negative patients, there were 52 patients without tumor, 63 with adenoma, 27 with adenocarcinoma, and 90 with tumor. Among the H. py-lori-positive patients, there were 136, 264, 127, and 391 patients respectively. Pooling all subjects, those infected with H. pylori had a significantly increased OR for adenoma, adenocarcinoma, or tumor, compared to H. pylori-free patients (OR, 1.60, 1.80, and 1.66, respectively). For female H. pylori-positive subjects, the risk of having adenocarcinoma or tumor was significantly higher than that for their H. pylori-free counterparts, while for male H. pylori-positive and -negative subjects, there was no such significant difference. Conclusions: The results therefore suggest that, in patients aged 40-80 years, H. pylori infection increased the risk of colorectal adenoma and adenocarcinoma, with significantly higher risks for female patients. Background: Recent reports suggest that Helicobacter pylori infection may potentially increase the risk of colorectal cancer. The purpose of this study was to assess the association between H. pylori infection and the risk of colorectal adenoma and adenocarcinoma, and to evaluate any differences on the basis of sex. Methods: The subjects were 669 (40-to 80-year-old) patients who underwent both barium enema examination and total colonoscopy, and who were evaluated for H. pylori infection by 13C-urea breath test, urease test, or There were 142 H. pylori-negative and 527-positive patients. The odds ratios (ORs) for H. pyloripositive patients with colorectal adenoma and adenocarcinoma, and for tumor patients with either adenoma or adenocarcinoma were calculated. Results: Among the H. pylori-negative patients, there were 52 patients without tumor, 63 with adenoma, 27 with adenocarcinoma, and 90 with tumor. Among the H. py-lori-positive p atients, there were 136, 264, 127, and 391 individuals respectively. Pooling all subjects, those infected with H. pylori had a significantly increased OR for adenoma, adenocarcinoma, or tumor, compared to H. pylori-free patients (OR, 1.60 , 1.80, and 1.66, respectively). For female H. pylori-positive subjects, the risk of having adenocarcinoma or tumor was significantly higher than that for their H. pylori-free counterparts, while for male H. pylori-positive and -negative subjects, there was no such significant difference. Conclusions: The results therefore suggest that, in patients aged 40-80 years, H. pylori infection increased the risk of colorectal adenoma and adenocarcinoma, with significantly higher risks for female patients.