The health status of a 4-year-old female, dd-haplotype miniature pig deteriorated rapidly, so the animal finally had to be euthanized because of poor clinical condition. Necropsy revealed a massive leukocytic infiltration in the parenchymatous organs of the abdominal cavity. On hematologic cell counting, severe leukocytosis (69.3 x 10(9) cells/liter) and high-grade basophilia (6.9 x 10(9) cells/liter) were evident. Cytologic examination, as well as analysis of expression of leukocyte differentiation antigens by means of flow cytometry, classified blasts, which accounted for about 22% of leukocytes, as biphenotypic cells co-expressing the myeloid marker SWC3 (CD172a) and the lymphoid markers CD5 and CD25. Hematologic features resembled those seen in humans with chronic myeloid leukemia at blast phase.