t(11;18)(q21;q21), the most frequent chromosomal aberration of mucosa-associated lymphoid tissue (MALT) lymphoma, occurs in 30% of gastric patients. Although the translocation is often associated with an 'aggressive' course, it has not been described in transformed MALT lymphomas. We screened 15 gastric MALT lymphomas [three with concurrent or subsequent high-grade transformation and 11 diffuse large B-cell lymphomas (DLBCLs)] in Chinese patients for t(11;18). t(11;18) was found in 9/15 (60%) MALT lymphomas, but not in any DLBCLs. One patient, with subsequent high-grade transformation, showed the translocation in low- and high-grade lesions. t(11;18) was frequent in Chinese gastric MALT lymphomas and unusually one transformed lymphoma carried the translocation.