We present a case of myelodysplastic syndrome (MDS) following treatment for medulloblastoma. The tumor was diagnosed at 15 years of age and managed with surgery, craniospinal radiation and a triple-drug regimen [chloroethylnitrosourea (CCNU), vincristine (VCR), procarbazine]. Fifteen months after the completion of therapy, the patient developed MDS with monosomy 5 and monosomy 7 on chromosome analysis of bone marrow cells. Two weeks later, MDS evolved into acute myeloblastic leukemia (AML). Therapy-related MDS or AML may be a complication for patients with past brain tumors. Therefore, they should be carefully followed up by regular physical examinations and complete blood counts.