The c-myc gene is involved in important cellular processes, including cell proliferation, differentiation, and apoptosis. We analyzed mutation of the c-myc gene in 51 patients with gastric lymphoma [27 patients with low-grade mucosa-associated lymphoid tissue (MALT) lymphoma, 11 with high-grade MALT lymphoma, and 13 with diffuse large B-cell lymphoma (DLL)], by polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) analysis. We also evaluated the relationship between mutation of the c-myc gene and regression of low-grade MALT lymphoma after Helicobacter pylori (H. pylori) eradication. Mutation in exon 2 of the c-myc gene was present in 2 of 20 (10%) patients with low-grade MALT lymphoma, in 1 of 7 (14%) patients with high-grade MALT lymphoma, and none of 10 patients with DLL. The 3 patients who had mutations of the gene, showed different patterns of mobility shift, suggesting different mutations. In addition, 15 patients with low-grade MALT lymphoma received anti-H. pylori therapy. All the patients achieved eradication. Nine of the 15 (60%) patients with low-grade MALT lymphoma showed complete regression (CR), 3 (20%) showed partial regression (PR), and 3 (20%) showed no change (NC). One of the 9 (11%) CR patients had a mutation of the c-myc gene. None of the 3 PR and 3 NC patients had mutation of the gene. There was no significant difference between the frequencies among the c-myc gene mutation in CR, in PR and in NC patients. These data suggest that mutation of the c-myc gene may not be commonly associated with development of gastric MALT lymphoma and DLL, and may not be associated with regression of low-grade MALT lymphoma after H. pylori eradication.