Two siblings developed a fulminant fatal myeloproliferative disease at 7 and 8 weeks of age. The illness presented with pallor, haemorrhagic symptoms, and hepatosplenomegaly, and the blood picture was that of pancytopenia and leucoerythroblastosis. Bone marrow histology showed reduced haemopoiesis with generalised fibrosis. Histiocytes were present, but haemophagocytosis was not prominent. There was evidence of extramedullary haemopoiesis in the spleen, with a chronic inflammatory infiltrate of other organs. The condition closely resembles acute idiopathic myelofibrosis of infancy, but the early onset with severe pancytopenia and the histological appearances may arouse suspicion of the possible familial nature of the condition. Although clinically resembling familial haemophagocytic reticulosis, the uncharacteristic bone marrow, liver, and spleen histology serve to exclude this diagnosis.