The data of detailed studies of immunophenotype of blast cells in 426 patients with acute leukemias are presented. Diagnostic and prognostic significance of different marker expression has been evaluated in groups of patients with ALL and AML. Frequency distribution of T1, T2, T3, pre-B, B, common, Ia and null subvariants identified according to immunoclassification of Baryshinkov et al. was studied in 250 children with ALL. These subvariants differed both in duration of disease (p = 0.0015) and in duration of first complete remission (p = 0.0031). The use of monoclonal antibodies of VI-series in 90 patients with ALL allowed to describe an immunophenotype of the subvariants in detail. The mosaic expression of myeloid antigens CD11, CD14, CD15, CDw65 identified by MoAbs VIM-12, VIM-13, VIM-D5 and VIM-2, respectively, on blast cells of patients with AML was shown. The expression of CD11 (ICO-GM1) or CD15 (ICO-G2) was prognostically unfavorable in children with AML (p = 0.0028). The expression of T-cell markers (E-receptor, CD7, reactivity with anti-T-cell serum) on blasts was prognostically favorable in children with AML (p = 0.003). So the data of immunophenotyping are of great value for accurate diagnosis and prognosis of acute leukemias.