Acute gastric mucosal lesions (AGML) comprise a typical clinical entity in patients with acute gastritis, which is characterized by severe erosion, hemorrhage, and ulceration. It is thought that most Helicobacter pylori (H. pylori) infections are established during childhood through human-to-human contact. Initial H. pylori infection in an adult is rare, and the transmission route is unknown. The first patient was a 27-year-old woman whose chief complaint was epigastric pain. She underwent dental treatment for 30 min and developed sudden epigastric pain 6 h after the treatment. Endoscopic examination revealed multiple hemorrhagic erosions in the antrum. Rapid urease test and histology for H. pylori were positive, but serum anti-H. pylori IgG antibody was negative at the onset. Serology and urea breath test were positive for H. pylori 2 months after the dental treatment. The second and third patients were diagnosed as having AGML 2 and 4 days after dental treatment, respectively. Culture for H. pylori was positive and serology was negative at the onset, but serology showed seroconversion 2 months after the dental treatment in both patients. These findings indicate that dental treatment is a possible route for H. pylori infection in patients with AGML.