The characteristics of de novo diffuse large B-cell lymphoma (DLBCL) with translocation of c-myc and immunoglobulin (Ig) genes (c-myc/Ig DLBCL), were investigated in 13 cases of c-myc/Ig DLBCL. Immunohistochemistry revealed five cases were positive for CD10 and BCL6 expression (CD10(+)/BCL6(+)), five cases of CD10(-)/BCL6(+)/MUM1(-), one case of CD10(-)/BCL6(+)/MUM1(+) and two cases of CD10(-)/BCL6(-)/MUM1(+) expression, indicating 10 cases of germinal center B-cell DLBCL and three cases of non-germinal center B-cell DLBCL. Ongoing mutation of the Ig heavy chain gene variable region (IgH-V) was found in two cases with CD10 and BCL6 expression and one case showing CD10(-)/BCL6(+)/MUM1(-) expression. These three cases of ongoing mutation of the IgH-V gene did not express BCL2, unlike those without ongoing mutation. These results suggest a heterogeneous immunophenotype and genotype for c-myc/Ig DLBCL, with CD10(-)/BCL6(+)/MUM1(-) cases the most frequent.