Leukaemoid reaction with megakaryocytic features in newborns with Down's syndrome.

A leukaemoid reaction was observed in 3 newborns with Down's syndrome. Thrombocytopenia was present in 2, requiring platelets transfusions in 1, and red cell transfusions were necessary in 2 patients. Blast cells characterization by specific monoclonal antibodies showed a prevalence of megakaryoblasts in all 3 cases. This feature was confirmed in 2 of them by the demonstration of platelet peroxidase (PPO) activity under transmission electron microscopy (TEM). A spontaneous remission of the leukaemoid picture was observed after 2-3 months. However, in 1 case a relapse of the myeloproliferative disorder with the same features of the blast cell population was diagnosed after 16 months. Chemotherapy with low-dose Ara-C, started because of a relevant clinical involvement, induced a complete remission.