The experience with myeloproliferative diseases (MPD) in children of a large pediatric hematology-oncology service during a 20-year period is reviewed. Twenty-seven patients with myeloproliferative diseases were treated, six with the juvenile type of chronic myelogenous leukemia (CML), 10 with the adult type of CML, three with familial MPD, one with unclassifiable MPD, and seven with acute leukemia in whom myelofibrosis was either a prodromal or terminal event. The literature is reviewed with particular emphasis regarding the relationships between the juvenile type of CML and monocytic leukemia, the adult type of CML and acute nonlymphocytic leukemia, and the relationship of myelofibrosis and myeloid metaplasia to the acute leukemias. New therapeutic approaches are needed in this heterogenous but interrelated group of disorders.