Clinical bone marrow specimens submitted to the University of Minnesota's Veterinary Teaching Hospital Cytology Service over a 3-year period were evaluated for the presence of myelodysplastic features. Of 220 bone marrow specimens examined, 30 contained dysplastic features. Twenty-seven of these dogs were evaluated further. Twelve were categorized as primary myelodysplastic syndromes, and 15 were categorized as secondary myelodysplastic syndromes. Of the primary myelodysplastic syndromes, 4 were subcategorized as refractory anemia and 8 were categorized as myelodysplasia. Primary refractory anemia was characterized by nonregenerative anemia without leukopenia or thrombocytopenia and with prolonged survival. Primary myelodysplasia was characterized by pancytopenia, greater than 5% myeloblasts in bone marrow, dysplastic features in all bone marrow cell lines, and short survival time. Results of this study indicate that differentiating primary refractory anemia from primary myelodysplasia has both therapeutic and prognostic significance. Dogs with primary refractory anemia tend to have prolonged survival and respond to erythropoietin treatment, whereas dogs with primary myelodysplasia have short survival and do not respond to standard treatments.