Since 1997 we have experienced three cases of low-grade colonic mucosal-associated lymphoid tissue (MALT) lymphomas. The depth of tumor invasion was evaluated by endoscopic ultrasonography (EUS) and the mass lesions were all diagnosed as having extended beyond the deep region of the submucosal layer. Although all of these patients tested negative for gastric Helicobacter pylori, their tumor lesions regressed after antibiotic treatment in accordance with H. pylori eradication therapy. In general, consensus has been reached regarding antibiotic therapy for gastric MALT lymphomas. However, as a prerequisite for antibiotic therapy, the therapy has been deemed effective against these gastric tumors if the extent of infiltration, as evaluated by EUS, is limited to the mucosal layer or the superficial region of the submucosal layer. Based on the therapeutic outcomes seen in the three patients studied here, it is suggested that antibiotic therapy might be useful in treating MALT lymphomas of the colon, even in patients with advanced invasive tumors, in contrast to the extent of the lesions in the stomach suitable for antibiotic treatment. The success of the antibiotic treatment also suggests that MALT lymphomas may be caused by unknown luminal microorganisms, other than H. pylori. Since 1997 we have experienced three cases of low-grade colonic mucosal-associated lymphoid tissue (MALT) lymphomas. The depth of tumor invasion was evaluated by endoscopic ultrasonography (EUS) and the mass lesions were all diagnosed as having extended beyond the deep region of All of these patients tested negative for gastric Helicobacter pylori, their tumor lesions regressed after antibiotic treatment in accordance with H. pylori eradication therapy. In general, consensus has been reached regarding antibiotic therapy for gastric MALT lymphomas. However, as a prerequisite for antibiotic therapy, the therapy has been effective against these gastric tumors if the extent of infiltration, as evaluated by EUS, is limited to the mucosal layer or the superficial region of the submucosal layer. Based on the therapeutic outcomes seen in the three patients studied here, it is suggested that antibiotic therapy might be useful in treating MALT lymphomas of t he colon, even in patients with advanced invasive tumors, in contrast to the extent of the lesions in the stomach suitable for antibiotic treatment. The success of the antibiotic treatment also suggests that MALT lymphomas may be caused by unknown luminal microorganisms, other than H. pylori.