Thirty-three children diagnosed with primary myelodysplastic syndrome (MDS) in a single institution over an 8 year period were evaluated with special emphasis on children who presented with extramedullary disease (EMD). EMD was present at diagnosis in 12 (36%) of the 33 children with MDS. Three patients with juvenile myelomonocytic leukemia (JMML) and 2 patients with chronic myelomonocytic leukemia (CMML) presented with pleural effusion, Pericardial effusion was present in 3 of these patients, two of whom also had thrombosis, Pyoderma gangrenosum, relapsing polychondritis were the initial findings in another two cases with JMML. Lymphadenopathy (n=1), gingival hypertrophy (n=2), orbital granulocytic sarcoma (n=1) and spinal mass (n=1) were the presenting findings in 5 patients with refractory anemia with excess of blasts in transformation. Since high-dose methylprednisolone (HDMP. 20-30 mg/kg/day) has been shown to induce differentiation and apoptosis of myeloid leukemic cells in children with different morphological subtypes of acute myeloid leukemia in vivo and in vitro, 25 children with de novo MDS were treated with combined HDMP and cytotoxic chemotherapy. Dramatic improvement of EMD and decrease in blast cells both in the peripheral blood and bone marrow were obtained following administration of short-course HDMP treatment alone as observed in children with AML. HDMP, combined with low-dose cytosine arabinoside and mitoxantrone were used for the remission induction. Remission was achieved in 8 (80%) of 10 children who presented with EMD and in 9 (60%) of 15 children without EMD. Long-term remission (>6 years) was obtained in 4 (two with JMML and two with CMML), three of whom presented with EMD.