Purine metabolism enzyme pattern, cytochemical markers and clinicopathologic features of common acute lymphoblastic leukemia antigen (cALLA; CD10)-positive, CD10-negative T acute lymphoblastic leukemia (ALL), and cALLA-positive non-T, non-B ALL (common ALL; C ALL) of children were compared. The results of immunophenotyping of blast cells in 61 children with ALL who were treated and followed during the last 7 years at the Second Pediatric Clinic in Bratislava are presented. The aim of our study was to determine the correlation of CD10 marker expression with purine enzyme activities and clinical course in ALL of children. Immunologic phenotype performed by a panel of monoclonal antibodies in indirect immunofluorescence assay revealed 3 main ALL groups: Common ALL (C ALL), T ALL and CD10+ T ALL (C + T ALL). An additional exact cytochemical marker analysis was performed in these three ALL immunologic subtypes. Two enzymes of purine metabolism, i.e. adenosine deaminase (ADA) and purine nucleosidephosphorylase (PNP) were investigated in blast cells by paper radiochromatography. Life-table analysis revealed significant prognostic differences with regard to event-free survival and overall survival in followed groups of ALL patients. Our results showed a rather high frequency of mixed (C + T) ALL phenotype. The characteristic T ALL enzyme pattern (high ADA, low PNP) was present not only in T, but also in CD10+ T ALL blast cells. The T cell marker showed to be dominant in the determination of clinical course and prognostic significance in children with ALL; children with T and CD10+ T ALL phenotype, in contrast to C ALL phenotype, experienced more frequent relapses and a shorter event-free survival.