Recently, the water-soluble bifunctional alkylating agent treosulfan demonstrated broad stem cell toxicity, immunosuppressive as well as antileukemic activity. Due to its well known low non-hematologic toxicity profile, treosulfan was considered an alternative agent for conditioning prior to allogeneic transplantation. A first clinical study, combining 3 x 10 g/m2 of treosulfan with 5 x 30 mg/m2 of fludarabine, demonstrated the feasibility of this conditioning. A fast, reliable and complete development of the donor hematopoiesis was evident as well as a low non-hematologic toxicity, transplantation-related mortality and relapse rate. In a second study treosulfan was escalated from 3 x 10 to 3 x 12 and 3 x 14 g/m2. In this protocol, 55 pts (patients) not amenable to standard conditioning suffering from various hematological malignancies were included. Complete donor chimerism was reached by day 28 in 80% of the pts. So far, 8 pts (11%) died without disease progression and 11 pts (20%) relapsed. Treosulfan was very well tolerated. Especially no hepatic VOD, severe cardiac or pulmonary toxicity was noted. Acute GvHD (degrees 11-IV) occurred in 44% and chronic GvHD in 45% of pts. Considering the poor prognosis of these study populations, treosulfan-based conditioning is considered to be safe and efficient. New phase 11 clinical protocols in AML and MDS will be initiated.