Investigators hypothesize that combination of ATLG with PTCy in matched or mismatched unrelated hematopoietic stem cell transplantation will reduce acute and chronic GVHD incidence. Furthermore it will allow shortening of the length of post-transplantation immunosuppression with calcineurin inhibitor (CNI) administration (currently administrated in addition to ATG as GVHD prophylaxis in daily common practice)

Inclusion Criteria:

1. Patients with MDS/AML
2. 18 years or older and willing and able to comply with the protocol requirements.
3. LVEF ≥ 40%. 2-D transthoracic echocardiogram (ECHO) is the preferred method of evaluation. Multigated Acquisition Scan (MUGA) is acceptable if ECHO is not available.
4. Patients undergoing 8-10/10 HLA matched unrelated and unmanipulated PBSC transplantation
5. Patients conditioned with reduced intensity or reduced toxicity conditioning of fludarabine with reduced dose (2 days) or myeloabalative doses (4 days) of busulfan or with treosulfan.
6. Patients must sign written informed consent.
7. Adequate birth control in fertile patients.

Exclusion Criteria:

1. Patients undergoing other type of transplantation or with other type of basic disease other than AML or MDS.
2. Patients with respiratory failure (DLCO < 30%).
3. Active congestive heart failure (New York Heart Association [NYHA] Class III to IV), symptomatic ischemia, or conduction abnormalities uncontrolled by conventional intervention.
4. Psychiatric conditions/disease that impair the ability to give informed consent or to adequately co-operate
5. Bilirubin > 3.0 mg/dl, transaminases > 3 times upper normal limit
6. Creatinine > 2.0 mg/dl
7. ECOG-Performance status > 2
8. Uncontrolled infection
9. Pregnancy or lactation
10. CNS disease involvement
11. Pleural effusion or ascites > 1 liter.