Abstract: Objective : To study the clinicopathologic and immunohistochemical features and IgH rearrangement of primary thyroid MALT lymphoma. Methods : Specimens from 12 cases of primary thyroid extranodal marginal zone B-cell lymphoma were studied by immunohistochemical staining for CD20, CD45RO, CD5, CD10, CyclinD1, bcl-2, immunoglobulin light chain κ and λ and investigated for IgH rearrangement by PCR technique. Results : According to the histological pattern, immuohisto-chemical features and PCR results, all patients were diagnosed as primary thyroid MALT lymphoma.There were two males and ten females with a median age of 63 years (range from 50 to 76 years). 12 cases were all positive for CD20. The tumor cells expressed restrictively immunoglobulin light chain λin 2 cases, κ in 2 cases. The tumor cells were positive for bcl-2 in 1 case. The tumor cells were negative for CD45RO, CD5, CD10 and CyclinD1 in all cases. Monoclonal IgH rearrangement were found in 8 of 12 cases (66.7V). Follow-up data were obtained in 11 cases, with 3 cases dead and 8 cases alive. Conclusion : Primary thyroid MALT lymphoma is an indolent lymphoma which is easily misdiagnosed.Pathologic features, immunohistochemistry and appropriate molecular techniques will be useful in the diagnosis and differential diagnosis of primary thyroid MALT lymphoma.