Background: Myeloablative stem cell transplants have a lower rate of relapse than reduced intensity regimens. Timed sequential busulfan (TSB) with fludarabine (Flu) is a promising myeloablative regimen for patients undergoing matched sibling (MSD) or unrelated (MUD) donor transplantation (HCT) with low non-relapse mortality (NRM) (Popat et al Lancet Haematology 2018), but was not tested in haploidentical (haplo). Also, whether this approach can be used with post-transplant cyclophosphamide (PTCy) in MSD, MUD and haplo HCT is unknown. To address these issues, we conducted a prospective phase II study. Methods: Patients with hematological malignancies with MSD, MUD or haplo donor were eligible. They received fixed doses of Busulfan(BU) 80mg/m2 either on day -13 and -12 (n=45) or on -20 and -13 (n=10). Then, Flu 40mg/m2 was given on day -6 to -2 followed by Bu dosed to achieve target area under the curve (AUC) of 20,000 umol/min for the whole course based on pharmacokinetic studies. Thiotepa 5mg/kg was given on day -7 to haplo group. GVHD prophylaxis was PTCy 50mg/kg on day 3 and 4 and tacrolimus. Haplo and later MUD recipients also received mycophenolate mofetil. Results: 55 patients with a median age of 47 (15-65) years were enrolled. 30 patients had AML or MDS, 9 CML or MPD, 5 lymphoma, 5 myeloma and 6 ALL. About half had haplo 26 (47%); others had MUD 18 (33%) or MSD 11(20%). Disease risk index was high in 18 (32%), intermediate in 32 (58%), and low in 5 (9%) patients. Comorbidity score was ≥3 in 22 (40%) patients. With a median follow up of 17 months (5-28), 1-year OS, PFS, NRM and relapse rates were 71% (60-84%), 63% (51-77%), 20% (9-31%), and 17% (7-27%), respectively [Table]. There were no graft failures. Day 100 grade II-IV and III-IV acute GVHD rates were 38% (25-51%) and 9% (95% CI 1-17%), respectively; 1-year chronic GVHD and extensive chronic GVHD rates were 10% (2-28%) and 8% (0-15%), respectively. 1-year OS in MSD, MUD and haplo groups were 91% (75-100%), 72% (54-96%), and 62% (45-83%) respectively (P=0.11). Conclusions: Myeloablative TSB with PTCy is feasible in MSD, MUD and haplo HCT. It lowers the incidence of severe acute and chronic GVHD without apparent increase in relapse. Clinical trial information: NCT02861417