CD10 and MUM1 are representative B cell differentiation markers. Follicular lymphoma (FL) is usually positive for CD10 and negative for MUM1. In this study, however, we compared 22 FLs with peculiar phenotype CD10-MUM1+ with 119 typical CD10+MUM1- FLs. All CD10-MUM1+ FL patients exhibited follicular structure with follicular dendritic meshwork, and a high rate of somatic hypermutation and ongoing mutation, similar to typical FL. However, CD10-MUM1+ FLs were encountered frequently in the elderly compared with CD10+MUM1- typical FLs (67.0 versus 58.7 years, P < .01), showed high grade (grade 3A or 3B) morphology (91% versus 17%, P < .001), diffuse proliferation (59% vs 19%, P < .001), and lacked BCL2/IGH translocation (5% versus 92.5%, P < .001), which is the most characteristic aberration in FL, and 88% showed BCL6 gene abnormalities (translocation or amplification). Our results indicate that CD10-MUM1+ FL is different from typical FL with respect to biologic and clinical features.