The Tokyo Children's Cancer Study Group (TCCSG) and the Kyushu Yamaguchi Children's Cancer Study Group (KYCCSG) performed a collaborative analysis of data on children with Down syndrome and acute lymphoblastic leukemia (DS-ALL). Among the 1,139 patients who were enrolled in the TCCSG L99-15, L99-1502, or the KYCCSG ALL 96 study, 13 patients with newly diagnosed ALL had DS. In the DS patients, a significantly higher proportion of patients developed ALL at age 5 years or older compared with the non-DS ALL patients (P < 0.001). The 5-year relapse-free or overall survival of DS-ALL patients was 50.0 or 61.5%, respectively. Relapse accounted for all causes of death. In the TCCSG L99-15 cohort, the overall survival of DS-ALL patients was 42.9%, which was significantly worse compared with 87.9% in the non-DS population (P < 0.001). The survival of patients who received reduced-dose chemotherapy was significantly worse than those who received full-dose chemotherapy (P < 0.001). However, a higher dose of methotrexate was not associated with a better outcome. Results of our preliminary study suggest that the survival of DS-ALL patients could be improved by treatment without dose reduction if possible, although the appropriate dose of methotrexate for DS-ALL needs to be determined.