t(11:8) is a recurrent chromosomal abnormality observed in mucosa-associated lymphoid tissue (MALT)-type lymphoma. API2 and MLT genes have been implicated. The authors devised a dual-color interphase fluorescence in situ hybridization (FISH) system to detect splitting of 11q22 and its fusion with 18q21. Subjects were 44 cases of extranodal lymphoma and cases of primary macroglobulinemia. Whenever RNA was available, reverse transcriptase-polymerase chain reaction followed by sequence analysis was performed. Positive cases by dual-color FISH analysis were restricted to MALT-type lymphoma and one case of primary macroglobulinemia. Among 24 cases of MALT-type lymphoma, 14 (58%) (4 gastric, 5 pulmonary, 3 orbital, 1 salivary, and 1 thyroid lymphomas) had splitting of the 11q22 region probes and fusion of signals suggesting the translocation of chromosome 11 and 18. Reverse transcriptase-polymerase chain reaction analysis showed the API2/MLT gene fusion in 9 of 10 cases. Sequence analyses showed three different modes of involvement of the MLT gene, whereas the breakpoint at API2 was the same. Monoclonal component of serum immunoglobulin M was observed in 3 of 14 positive cases for the translocation. Direct visualization using dual-color FISH on samples serves as a molecular tool for management of MALT-type lymphoma with API2/MLT gene fusion.