MUM1/IRF4 expression is detected in 18% to 41% of Burkitt lymphoma (BL). However, only a few studies of MUM1-positive (MUM1) BL have been reported, and its characteristics still remain controversial. To highlight the features of MUM1 BL, we compared the clinicopathologic characteristics of 37 cases of MUM1 and 51 cases of MUM1-negative (MUM1) BL in Japan. Compared with MUM1 BL, patients with MUM1 BL showed significantly younger onset (P=0.0062) and a higher ratio of females (P=0.013). We have also revealed the difference in the involved sites. The MUM1 group showed lower incidences of involvement of stomach (P=0.012) and tonsil (P=0.069). There was a more tendency in MUM1 group to involve colon (P=0.072), breast (P=0.073), and kidney (P=0.073). Regarding the prognosis, a trend toward a lower overall survival for MUM1 group was noted (P=0.089). Notably, comparing MUM1 and MUM1 BL cases of adults (age16 y old and above), the former showed significantly worse prognosis (P=0.041). Among the BL patients treated with the intensive chemotherapy, a standard therapy for BL, MUM1 cases showed worse prognosis (P=0.056). In conclusion, MUM1 BL showed worse prognosis, particularly in adult cases, compared with MUM1 BL. In addition, the difference of the onset age, sex ratio, and involved sites between the 2 groups was noted. Our results demonstrate that MUM1 expression might predict worse prognosis of BL, and MUM1 BL should be distinguished from MUM1 BL.