Few cases of secondary CD5+ diffuse large B-cell lymphoma (DLBCL) that are not Richter syndrome have been reported previously. We report 9 cases of non-Richter syndrome secondary CD5+ DLBCL. Among 529 cases of DLBCL, 38 (7.2%) were CD5+ DLBCL, including 9 of secondary CD5+ DLBCL. Five cases gained CD5 expression during the clinical course of DLBCL (group 1). Three cases showed transformation from CD5- low-grade B-cell lymphoma to CD5+ DLBCL (group 2). The remaining case showed coexistence of CD5+ DLBCL and CD5+ follicular lymphoma. The clonal relationships of CD5- and CD5+ tumors were confirmed in all 4 available cases. Cases of secondary CD5+ DLBCL that were not Richter syndrome were classifiable into 3 groups. Groups 1 and 2 showed the gain of CD5 during the clinical course or transformation of the tumors, suggesting that CD5 expression is closely associated with the progression of B-cell lymphoma.