Abstract： Objective To clarify the MUM1/IRF4 expression in follicular lymphoma (FL) and its clinical and pathological significance. Methods Ninety-six cases FL were immunostained with MUM1,CD10,bcl-2,bcl-6 and Ki-67 antibodies. The results were compared with their clinical and pathological features. Results The overall MUM1 expression rate in FL was 59.2 % (58/96),including 36.2 % (19/51) grade 1 or 2 and 86.4 %(39/45) grade 3 cases (x2 =24.406,P <0.001). 68.9 % cases with diffuse area were MUM1 positive (x2 =8.161,P =0.004). MUM 1 and CD10 expression had inverse correlation,83.3 % CD10 negative cases were MUM1 positive (x2= 12.649,P<0.001). The mitosis rate and Ki-67 label index were statistically higher in MUM1 positive cases than in negative cases (t = -3.852 & -4.610,respectively,P <0.001). Conclusion MUM1 can be used as a biomarker to divide FL into different malignancies. The MUM1 positive FL may be the feature of high grade non germinal center B cell malignant lymphoma.