A rare case of endoscopic and histological regression of a gastric lymphoid mucosal lesion after eradication of Helicobacter pylori is reported. A 72-year-old man was suspected of having a low-grade B-cell gastric mucosa-associated lymphoid tissue (MALT) lymphoma by endoscopic and histological findings. Histology of biopsy specimens showed massive infiltration of atypical lymphocytes and lymphoepithelial lesions. Immunohistochemical staining revealed kappa light chain expression in the infiltrated atypical lymphocytes to be twofold that of lambda light chain. The above diagnosis was thus highly suspected but not confirmed. Antibiotic therapy was given on the basis of evidence of H. pylori infection. Successful eradication of H. pylori resulted in remarkable improvement of endoscopic and histological findings. Follow-up studies were carried out 8 months after eradication, with no evidence of relapse. The eradication of H. pylori appears to be an effective alternative therapy for B-cell lymphoproliferative disease, although longer follow-up and further studies are needed before this treatment can be established.