To evaluate whether children with acute myeloblastic leukemia (AML) presenting with extramedullary infiltration (EMI) have different clinical, morphologic features and prognosis from children without EMI, a 127 consecutive previously untreated children with AML were entered in this study. Fifty-one children (40%) had EMI at diagnosis and 27% of these showed multiple site involvement. Twenty-seven of 127 children (21 %) presented myeloid tumors. No age related differences in the incidence of EMI was noted. However, analysis of clinical and biological features at diagnosis showed that WBC count greater than or equal to50 x 10(9) l(-1), hepatosplenomegaly >5 cm, FAB AML-M4 and AML-M5 subtypes and CD13, CD14 expression of bone marrow (BM) leukemic cells (>20%) were more frequent in children with EMI. Two consecutive treatment protocols were used. In both protocols remission was achieved with combined high-dose methylprednisolone (HDMP) as a differentiating and apoptosis inducing agent with mild cytotoxic chemotherapy (low-dose cytosine arabinoside (LD Ara-C), weekly mitoxantrone and Ara-C or 6-thioguanine). Administration of short-course (4-7 days) HDMP (20-30 mg/kg per day) alone resulted in a remarkable decrease in peripheral blood, BM blasts and in the size of EMI in responding patients. In both protocols, remission rate in patients with EMI was 71 and 80%, which was lower than that of the patients without EMI (87 and 89%). This may be attributed to the higher frequency of unfavorable features in children with EML However, in patients who presented with myeloblastoma and treated with a more intensive post-remission therapy (AML-94), the 4-year disease-free survival (DFS) and event-free survival (EFS) rates were not found to be significantly different from children who had no EMI (P > 0.05). Whereas, the outcome of children who presented with gingival infiltration did not improve. In further studies, the prognostic significance of different localisation of EMI and the effect of addition of HDMP to cytotoxic chemotherapy should be explored in larger series. (C) 2003 Elsevier Ltd. All rights reserved.