The article presents a clinical case of an unclassifiable myeloproliferative neoplasm in a 65-year old patient. Platelet count is not a criteria for differential diagnosis of reactive and tumor processes. A typical course of the disease is associated with the persistence of symptoms of microcirculation disorder against the background of previous asymptomic thrombocytosis for several years. The diagnosis of an unclassifiable myeloproliferative neoplasm is established by a combination of clinical, laboratory, instrumental, molecular genetic and morphological methods of research.