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.