Background: Acute leukemia is the most common malignancy in children and acute lymphoblastic leukemia (ALL) accounts for 75% of acute leukemia cases. New treatment protocols have resulted in complete remission rates up to nearly 100% in children with acute lymphoblastic leukemia. Today, one of the most important prognostic factors in acute lymphoblastic leukemia is intensity of the treatment. Risk stratification is accomplished based on clinical, morphological, immunophenotypic and cytogenetic findings. The aim of this study was to determine some prognostic factors in children with acute lymphoblastic leukemia. Methods: In this retrospective study information about age at onset of acute leukemia, sex, initial white blood cell count, FAB-subtype, immunophenotype, and clinical course of newly diagnosed acute lymphoblastic leukemia were extracted from medical records of children admitted to pediatric oncology department of Mofid Childrens Hospital from 1997 to 2000.Findings: There were 81 (58.3%) male and 58 (41.7%) female. 3.6% of patients were under 1 year old, 18.7% were above 10 years and 77.7% were 1-10 years old. Initial WBCs more than 50,000/ mm3 were observed in 24.5% of patients. 91.4% of patients showed FAB-subtype of L1, 7.9% of L2 and L3 was detected just in 0.7% of cases. Lymphoblasts were of B-cell lineage in 92.7% of patients, with pre-B cell in 74.8%, early pre-B cell in 17.1% and mature B cell in 0.8%. As a whole, complete remission was observed in 79.3% of the patients. 12.9% of patients had a relapse, second remission was achieved in 2.6% of relapsed cases. Conclusion: In this study, FAB-subtype L1 and pre-B cell immunophenotype was more common than previous studies. Other results were the same as reported in older studies.