Leukocytosis is occasionally seen in patients with presumptive but undiagnosed myeloproliferative disorders (MPD). A 74-year-old woman was admitted to our hospital for tarry stools, anemia, and marked peripheral leukocytosis of 1.4 x 10(5)/mu L. Gastroenteroscopy revealed an acute gastric and duodenal mucosal lesion that was treated successfully via endoscopic hemoclipping. Bone marrow aspiration revealed marked megakaryocyte proliferation with atypia of naked nuclei and marrow hypercellularity (90% cellularity). A fluorescence in situ hybridization test could not detect the BCR-ABL fusion gene. Bone marrow aspiration later revealed further abnormalities of megakaryocytes. The patient died from cerebral bleeding. The present case fulfilled 2 of the 3 major criteria of primary myelofibrosis according to the World Health Organization 2008 classification: namely, megakaryocytic hyperplasia with hypercellular marrow and granulocytic hyperplasia. However, the megakaryocytic abnormality was not strictly compatible with the criteria. Instead, we considered prefibrotic primary myelofibrosis as a possibility, although myelodysplastic syndrome/myeloproliferative neoplasm, unclassifiable (MDS/MPN-U) was technically the correct diagnosis. The present case shows that MPN diagnosis remains difficult and suggests that other cases of peripheral leukocytosis with diagnosed MDS/MPN-U might include similar findings.