The combination of fludarabine and melphalan as a reduced-intensity conditioning (RIC) regimen extends allogeneic hematopoietic SCT (HSCT) as a therapeutic option for elderly or frail patients with relapsed, refractory or other high-risk hematologic malignancies. Whether any modifiable factors exist that could improve survival before or immediately after HSCT is unknown. We reviewed the medical records of the first 50 patients at our institution to undergo fludarabine/melphalan RIC from September 2000 to September 2007 to determine factors associated with survival. A total of 25 (50%) patients had undergone prior HSCT and as such was a high-risk group of patients. On multivariate analysis, CD34(+) cell dose greater than 5.5 × 10(6) per kg (risk ratio (RR) 0.44, 95% CI 0.19-0.98, P=0.02) and full donor chimerism at day +100 (RR 0.17, 95% CI 0.06-0.64, P=0.002) remained independent prognostic factors. In our series, achievement of full donor chimerism at day +100 was associated with an approximately 70% 2-year survival, a favorable outcome in this high-risk group of patients. Although the infused CD34(+) cell dose is a modifiable variable, whether donor lymphocyte infusions or other immunologic interventions should be performed to promote the establishment of full chimerism early post transplant remains unknown.