e22001 Background: Acute lymphoblastic leukemia (ALL) is the most common malignancy of childhood. With the improvements in effective antileukemic drugs and the risk stratification treatment approach, the cure rate for pediatric ALL has increased to >90%. The effect of outpatient treatment and developing supportive care have a very important role in this success. We evaluated the disease outcomes and severe complications (complication causes hospitalization in ICU, encephalitis) of children who had been treated with the Children’s Oncology Group (COG) protocols, and follow-up protocol during a 20-year study period. Methods: From 1999 to 2020, 134 patients who had been diagnosed with ALL in the Department of Pediatric Hematology-Oncology at the Istanbul University Oncology Institute were included in the present study. Nine patients excluded from all analysis that 5 of them admitted with relapse, 2 had been diagnosed with mature aggressive B-cell acute leukemia, and 2 had been diagnosed with mixed phenotype and 7 who had been lost to follow-up were also excluded from survival analyses. Results: Mean age was 7.7±4.89 years old in 125 patients (Table). The 5- and 10-year and 20-year Event Free Survival (EFS) rates were 90.2% (standard error [SE], 4.2%) in standart risk B-ALL (n=54) and Overall Survival (OS) rates were 100%, 96% and 96% ([SE], 3.9%), respectively. In high risk group B-ALL (n=53) 5-year EFS was 80.7% (standard error [SE], 5.8%) and 78.2% ([SE], 6.1%) for both 10 and 20 year period and OS rates were 76.9% ([SE], 5.9%) for all periods. Although the number of patients is low in T-ALL (n=11), 5- and 10-year and 20-year EFS and OS rates were 63.6% (standard error [SE], 14.5%) for all periods. Fifteen severe complication recorded in 12 patients (10 B-ALL, 2 T-ALL). One patient hospitalized in ICU due to gram negative sepsis for 3-times and epidural hemorrhage. Conclusions: The survival rate for pediatric ALL has increased in recent years according to improvements of treatment.In our cohort, 20-year OS rate is 96% for standart risk B-ALL and 76.9% for high risk B-ALL. Additionally, regularly and closely monitored patients, complications can be effectively prevented, treated, and eliminated. In our study, the short hospitalization period, rapid analysis of laboratory tests, continuous supportive care, efficient education given to the parents of children increased the success of the therapy and ensured low complication rates. [Table: see text]