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Prolonged pharmacologic immunosuppression is a major obstacle to early immunologic recovery after allogeneic BMT. Based on our results in animal models, we studied whether properly timed high-dose Cy post-HLA matched related and unrelated BMT is an effective strategy for limiting GVHD; we hypothesized that avoiding prolonged immunosuppression would speed immune recovery and reconstitution of regulatory T cells (T regs) thereby decreasing post-transplant complications. We are reporting results on 46 consecutive patients (median age 41, range 1–64) with high-risk hematologic malignancies (20 AML, 12 ALL, 6 NHL, 3 HD, 2 MM, 2 CML, 1 CMMoL); 28 received related and 18 unrelated unmanipulated HLA-matched BM (median of 2.2 x 108 MNC per kg) after conditioning with busulfan on days -7 to -3 and Cy (50 mg/kg/day) on days -2 and -1, and followed by Cy (50 mg/kg/day) on days +3 and +4 as the sole GVHD prophylaxis. All the patients had advanced disease (20 in advanced remission with the rest having refractory disease), and the median follow-up is 13 (range 6–24) months. All but two patients had sustained engraftment. The cumulative incidence of acute grades II–IV and grades III–IV GVHD were 41% and 9%, respectively. All patients with GVHD responded fully to standard therapy (steroids ± tacrolimus) or therapy per BMT CTN0302, and all except 2 patients were rapidly weaned from all immunosuppressive agents. Of the thirty-six patients alive after day 100, only 1 of the 23 patients that received HLA-matched related, and 3 of 13 patients that received unrelated allografts, developed chronic GVHD. Twenty-six (56%) patients are alive, of whom 21 (45%) are in complete remission. There were no deaths secondary to infection or GVHD. CMV reactivation was detected in 11 of 36 (31%) patients, of whom 9 had GVHD. There was no CMV infection. Median (± SEM) CD4+ T cell counts were 99 ± 16/mL and 209 ± 49/mL on days 60 (n = 23) and 180 (n= 8), respectively. Corresponding values for CD8+ T cells were 248 ± 132/mL and 228 ± 161/mL on days 60 and 180, respectively. Patients with grade II–IV GVHD had significantly fewer peripheral blood (PB) CD4+Foxp3+ T cells compared to patients with grade 0–I GVHD (p