Lymphoid neoplasms in 17 cattle having tested positive for bovine leukosis virus (BLV) were investigated by histology and immunohistochemistry. In 16 cases, the neoplastic cells were characterized by a marked variation in cell size, atypia in large cells, nuclear irregularity in smaller cells, and nucleolar prominence in cells with scant cytoplasm. These cytological features were used to specifically define pleomorphic lymphomas. In addition to lymphoid cells, immunoblastoid and plasmacytoid cells containing cytoplasmic immunoglobulin G were observed in the remaining case. Since a similar case was recorded as lymphoplasmacytoid lymphoma in a BLV-negative cow, this polymorphic case may not be related to the virus. Most lymphoma cases associated with BLV are classified as pleomorphic, and neoplasms termed enzootic leukosis should be diagnosed based on tumor cell morphology.