The current outcome of children with acute lymphoblasic leukemia (ALL) after receiving risk-directed therapy has been substantially improved. Approximately 70- 80% of these children survive for more than 5 years without disease recurrence. But a proportion of these patients experience relapse. Monitoring of minimal residual disease (MRD) is an essential adjunct to early prediction of relapse and further tailored therapy based on identification of persistent disease. The objectives of this study are to establish incidence of MRD in Thai children with ALL during and after remission induction therapy, and to correlate the incidence with other standard prognostic parameters such as age, sex, white blood cell count, and DNA ploidy. Fifty-six bone marrow samples (BMs) (day15 and day43) from 32 consecutive children with ALL admitted and treated at Ramathibodi hospital were assessed for MRD based on leukemia-associated immunophenotypes using sensitive, rapid, and reproducible flow cytometric analysis (FC) which can detect 1 leukemic cell in 104 normal bone marrow cells. A high incidence of MRD positive at day15 and day43 was found in patients in high-risk group (66.7% and 33.3%, respectively). In comparison, only 14.3% and 9.1% of standard- and low-risk patients at day15, and 9.1% and 0% at day43 had detectable MRD. The multivariate statistical analysis showed no correlation of MRD to sex, age, immunophenotype, white blood cell count, DNA ploidy, Philadelphia chromosome, and the risk group. (P>0.007). Monitoring of MRD in Thai childhood ALL by FC method could provide insights and information on the responsiveness to treatment of the disease. Prognostic importance of the levels of MRD and its cut-off point for risk stratification requires further studies with larger sample size and long-term follow-up.