B‐Cell chronic lymphocytic leukemia (B‐CLL)/small lymphocytic lymphoma (SLL) consists of heterogeneous diseases that are distinguished by morphological, immunophenotypic and molecular features. MUM1 (multiple myeloma oncogene 1) is a protooncogene that is deregulated as a result of (6;14)(p25;q32) chromosomal translocation in multiple myeloma, and is also expressed in a variety of malignant lymphoma entities. We examined the expression of MUM1 in B‐CLL/SLL, and found that 2 of 4 B‐CLL‐derived cell lines and 14 of 29 patients' specimens expressed MUM1 by immunohistochemical analysis. MUM1 expression was not associated with CD38 expression, somatic hypermutation of immunoglobulin heavy chain gene variable region (IgV_H), or any other clinical characteristics of the patients. Interestingly, the patients who were positive for MUM1 showed shorter overall survival tunes than those who were negative for MUM1 (50% survival: 22 months vs. 82 months) (P=0.0008, log‐rank test). Multivariate analysis by Cox's proportional‐hazards regression model showed that MUM1 expression and unmutated IgV_H status were independent unfavorable prognostic factors in patients with B‐CLL/SLL. These findings suggest that MUM1 expression is a useful prognostic factor in B‐CLL/SLL. The biological role and mechanism of action of MUM1 in B‐CLL/SLL need to be clarified for the development of therapies for patients with the poor prognostic subtype.