The ultrastructural features of lymphoproliferative disorders from 18 dogs are described and compared with those recorded in the veterinary literature. A spectrum of diseases was selected for study to demonstrate the structural heterogeneity of these disorders. The most extreme variations were noted in nuclear contour, where bizarre cerebriform-appearing nuclei were characteristic of neoplasms of T-cell lineage, or single deep nuclear clefts suggested B-cell lineage. The lesions surveyed included acute and chronic lymphocytic leukemias, plasma cell neoplasms, cutaneous and multicentric malignant lymphomas, and cutaneous lymphoproliferative disorders with histologic similarities to human mycosis fungoides.