Aim: Mean Cell Volume (MCV) is an easily obtained parameter with the use of automatic blood counters in daily life and is mainly evaluated in the classification of anemias. Treatment protocols that include folic acid antagonists, DNA synthase inhibitors and cyclophosphamide may increase MCV. Our aim was to evaluate MCV changes of children with acute lymphoblastic leukemia under chemotherapy and to compare before and after treatment MCV values in relation to prognosis and relapse. Material and Method: One hundred-twenty-five children who were diagnosed as acute lymphoblastic leukemia after January 1995 in our centre were included in this retrospective study. The MCV values of patients (age 1-15 years, mean 5,99+3,94; 83 boys, 42 girls) at diagnosis, at induction/consolidation chemotherapy and after maintainence treatment were evaluated. Results: The MCV values of patients at diagnosis, at induction/consolidation chemotherapy and after maintainence treatment were 78,5+3,9 fL; 89,1+6,8 fL and 95,6+3,8 fL, respectively. On follow-up, MCV values were found to decrease with a mean of 1 fL per month after chemotherapy was completed. An ALL patient with concomitant thalassemia minor had a maxium MCV of 73 fL in spite of antimetabolite treatment. The results of 15 relapsed patients were compared with results of 110 patients without relapse, there was no difference in MCV values statistically (p>0.05). Conclusions: MCV was not found to be a significant parameter in relation to prognosis and relapse in children with acute lymphoblastic leukemia.