From 1982 to 1985, four cases of primary myelofibrosis were diagnosed in our department. Three were boys and one was a girl. Their ages ranged from 7 months to 15 years. The diagnosis was made based on anemia, leukoerythroblastic change and presence of giant platelets in the peripheral blood, and a bone marrow biopsy showing myelofibrosis. Most of them had anemia, fever, and hepatosplenomegaly on admission. The anemia was severe and refractory to repeated transfusions and steroid therapy in 3 out of the 4 cases. Splenectomy was performed in 1 case, but without satisfactory results. The clinical course and blood pictures in one case resembled leukemia of megakaryocyte lineage (M7), but results of marker studies of the blast cells ruled out the possibility of M7. Three of them underwent leukemic transformation within 2 years and died soon after. The other one died of sepsis 2 weeks after diagnosis. Myelofibrosis in childhood occurs rarely, however, when it does, it always runs a rapid and fatal course.