8514 Background: Limited information exists regarding risk of t-MDS/AML after Ewing’s sarcoma. Methods: We followed a cohort of 578 individuals less than 30 years of age, diagnosed with Ewing’s sarcoma, PNET or primitive sarcoma of bone between 1988 and 1994 (treated according to a therapeutic protocol: CCG-7881/POG-8850) to determine the incidence of t-MDS/AML and associated risk factors. Between 1988 and 1992, all patients were assigned randomly to standard chemotherapy with doxorubicin (dox), vincristine (VCR), cyclophosphamide (CPM), and dactinomycin (DACT) (Regimen A: n=262) or experimental therapy consisting of these four drugs in alternation with Ifosfamide (I) and etoposide (E) (Regimen B: n=256). Between 1992 and 1994, patients with metastatic disease were nonrandomly assigned to Regimen C (n=60). Cumulative therapeutic exposures were: Regimen A: CPM:20.4 g/2, dox:375 mg/m2; Regimen B: CPM:9.6 g/m2, dox:375 mg/2, E:5 g/m2, I:90 mg/2; Regimen C: CPM:17.6 g/m2, dox:450 mg/m2, E:5 g/m2, I:140 g/m2. Results: Median age at diagnosis (dx) was 12.0 yr. Eleven patients developed t-MDS/AML [Regimen A (n=3), Regimen B (n=2), Regimen C (n=6)] at a median of 3 yr., with a cumulative incidence of 2% at 5 years. Magnitude of risk by regimen is shown in the Table. Multiple regression analysis revealed that Regimen C (RR=15.9, 95% CI, 3.8–65.8), but not Regimen B (RR=0.6, 95% CI, 0.1–3.7) was associated with an increased risk of developing t-MDS/t-AML when compared with Regimen A. Age at diagnosis, gender, and use of radiation therapy for local control did not increase the risk of t-MDS/AML. Conclusions: The risk of t-MDS/AML after Ewing’s sarcoma is not increased after exposure to ifosfamide at a dose of 90 g/m2, or etoposide at 5 g/m2, when compared to those not exposed to ifosfamide or etoposide, but that the risk increases dramatically after exposure to ifosfamide at cumulative doses of 140 g/m2. No significant financial relationships to disclose.