Diffuse large B-cell lymphoma (DLBCL) is the most common type of lymphoma in dogs and humans. DLBCL has an aggressive and heterogeneous biologic course. Ki67 is an important marker of cell proliferation in humans with DLBCL. Mitotic index (MI) is another important indicator. Lymph node samples (n = 29) from dogs with DLBCL treated with a 19-week CHOP protocol were evaluated for Ki67 immunostaining and MI values. This study showed that Ki67 immunostaining on biopsy specimens was associated with shorter survival times in dogs with DLBCL. MI was not a prognostic factor, according to this study.