A group of conditions closely related to leukaemia in which one or more type of blood cell is overproduced by the bone marrow. There are three main types. In essential thrombocythaemia, too many platelets are produced. In polycythaemia vera, too many red blood cells are produced. In myelofibrosis, the bone marrow initially overproduces all types of blood cell, but then scarring of the bone marrow slows the blood cell production.