Immunophenotyping of bone marrow (BM) hemopoietic precursors is useful for diagnosis of adult myelodysplastic syndrome (MDS), but data concerning pediatric patients are limited. We analyzed immunophenotypic features of BM cells at diagnosis of children who were referred to the Brazilian Pediatric Cooperative Group of Myelodysplastic Syndromes. Methods: Diagnosis was based on clinical information, peripheral blood counts, BM cytology and cytogenetics. Patients with Down syndrome were excluded. Children with deficiency anemias or transitory neutropenias were used as controls (CTRLs). Immunophenotyping was performed on an eight-color antibody platform evaluatingmyelomonocytic maturation and progenitor cells. Results: Atotal of 32 patients were examined: 6 refractory cytopenia of childhood [RCC]; 5 refractory anemia with excess of blasts [RAEB]; 8 refractory anemia with excess of blasts in transformation [RAEB-t]; 13 juvenile myelomonocytic leukemia [JMML] and 10 CTRLs. Median age was 66 months (RCC), 68 months (RAEB/RAEB-t), 29 months (JMML) and 70 months (CTRLs). Median number of phenotypic alterations was 4 (range 1-6) in RCC; 6 (range 2-11) in RAEB/RAEB-t and 6 (range 2-11) in JMML (P = 0.004). The percentage of CD34(+)/CD117(+)/CD13(+) cells was 0.5% (range 0.1-2.8) in RCC; 4.2% (range 0.3-10.1) in RAEB/RAEB-t and 3.7 % (range 0.5-8.6) in JMML cases, compared with 0.7% (0.5-1.2) in CTRLs (P < 0.0005). Aberrancies in antigen expression of myeloid progenitors were seen in 63% of JMML and in 45% of RAEB/RAEB-t. CD34(+)/CD19(+)/CD10(+) cells were decreased or absent in patients compared with age-matched controls. T lymphocytes were decreased in JMML. Conclusions: Phenotypic abnormalities were similar to those found in adult MDS. A decrease in B-cell precursors was observed especially in RAEB/RAEB-t. JMML and RAEB showed a similar pattern.644Barretos Pediatric Cancer Center, BrazilBrazilian Research Council