Following 3 other centers in USA, Canada and Germany, we are evaluating fludarabine (40 mg/m2 on days −6 to −2) and busulfan (4 mg/kg/day on days −5 to −2) as a new conditioning regimen for allogeneic peripheral blood stem cell transplantation in leukemic patients with matched related donors. Seventy one patients were enrolled, 18 with high and 53 with standard risk (18 ALL, 35 AML, 16 CML and 2 MDS; F=29 M=42). The median patient age was 23.7 years(range, 2.4– 46.7). Cyclosporine was used as a prophylactic agent for GVHD (3mg/kg IV till +4, 10 mg/kg oral from day +5). The median follow-up was 269 days (range, 50–459 days). 91.5% and 15.5% developed mucositis and hepatic toxicity respectively which resolved with conservative therapy. There was no cardiac toxicity (except one patient with mild pericardial effusion and another with tachycardia). The median of highest serum creatinin level during hospitalization were 1.6 mg/dl (range, 0.8–3.7; 24.3% with Cr>2) and serum cyclosporine level, at the same time, was 246 ng/ml (range, 9–814). 7% experienced hemorrhagic cystitis (infection was ruled out) and 36.6% experienced moderate to severe headache. 38% and 14.1% of the patients showed grade 1, 2 and grade 3 acute GVHD respectively. Grade 4 acute GVHD was found in one patient. 50% and 6% showed limited and extensive chronic GVHD. 27% of patients became CMV+ (min +17, max +69). The median time for neutrophil and platelet recovery were 10 (min 0, max +26) and 12 (min 0, max +30) days. In day +38, 86.7% of the patients had 90% or more, mononuclear chimerism (with STR-PCR technique; median, 97%; range, 25–100). 5 ALL and 8 AML patients relapsed (18.3% of all patients) and 6 (8.45%) died after relapse. Nonrelapse mortality was 13% (9 patients; acute GVHD grade IV=1, CMV infection and GVHD=2, CMV infection=2, pneumonia=2, infection=2). With a median follow up of 9 months (range, 1.6–15.3 months), the probability of overall survival and disease free survival were 79.68% and 81.26% respectively. It could be beneficial to use fludarabine versus cyclophosphamide in standard conditioning regimen for leukemic patients because of reduced toxicity, low incidence of acute GVHD and facilitated donor engraftment.