Lymphoma of gastric-mucosa-associated lymphatic tissue (MALT) type has been linked to infection with Helicobacter pylori. We investigated the effect on MALT lymphoma of eradicating H pylori infection. 33 patients with primary gastric low-grade MALT lymphoma associated with H pylori gastritis were treated with omeprazole (120 mg daily) and amoxycillin (2.25 g daily) for 14 days to eradicate H pylori. In addition to histology, PCR was used to examine proliferation of monoclonal B cells before treatment and during follow-up. All patients had at least two post-treatment examinations, and all became negative for H pylori, 2 after a second treatment course. On histology, 23 (70%) patients showed complete regression and 4 (12%) partial regression of lymphoma. 6 (18%) patients had no change after cure of H pylori infection. 1 was treated with chemotherapy. Of 5 treated surgically, 4 were found to have high-grade B-cell lymphoma on histology of the resected stomach and 1 a high-grade T-cell lymphoma. PCR showed complete disappearance of monoclonal B cells after cure of H pylori infection in 13 of 16 patients investigated. During median follow-up of 1 year no relapse of MALT lymphoma occurred. Low-grade primary gastric MALT lymphoma can completely regress after eradication of H pylori infection. However, longer follow-up is needed to clarify whether the remission is lasting.