Myeloproliferative neoplasms are a group of diseases in which the bone marrow makes too many red blood cells, white blood cells, or platelets.

Normally, the bone marrow makes blood stem cells (immature cells) that become mature blood cells over time.

A blood stem cell may become a myeloid stem cell or a lymphoid stem cell. A lymphoid stem cell becomes a white blood cell. A myeloid stem cell becomes one of three types of mature blood cells:

  • Red blood cells that carry oxygen and other substances to all tissues of the body.
  • White blood cells that fight infection and disease.
  • Platelets that form blood clots to stop bleeding.

In myeloproliferative neoplasms, too many blood stem cells become one or more types of blood cells. The neoplasms usually get worse slowly as the number of extra blood cells increases.

There are 6 types of chronic myeloproliferative neoplasms.

The type of myeloproliferative neoplasm is based on whether too many red blood cells, white blood cells, or platelets are being made. Sometimes the body will make too many of more than one type of blood cell, but usually one type of blood cell is affected more than the others are. Chronic myeloproliferative neoplasms include the following 6 types:

  • Chronic myelogenous leukemia.
  • Polycythemia vera.
  • Primary myelofibrosis (also called chronic idiopathic myelofibrosis).
  • Essential thrombocythemia.
  • Chronic neutrophilic leukemia.
  • Chronic eosinophilic leukemia.

These types are described below. Chronic myeloproliferative neoplasms sometimes become acute leukemia, in which too many abnormal white blood cells are made.