Myelofibrosis with myeloid metaplasia is a chronic myeloproliferative disorder. The only curative therapy to date is myeloablation followed by allogeneic transplantation. This approach, however, is associated with a high morbidity and mortality. Autologous transplantation after myeloablation with high-dose oral busulfan provides a palliative approach which can lead to a long-term relief of symptoms and is associated with acceptable morbidity and mortality (Anderson JE et al. 2001, Blood 98: 586). However, busulfan pharmacokinetics after oral administration can vary between patients and increased toxicity is encountered in some. On the other hand treosulfan, a bi-functional alkylating drug, can be administered safely i.v. with reliable pharmacokinetics up to the myeloablative dose range. In the study presented here, patients were stimulated with G-CSF 16 μg/kg daily for 4 days and subsequent leukapheresis of a minimum of 5 x 106 CD34+ cells/kg was performed. Myeloablation consisted of treosulfan infusions of 14 g/m2 for three consecutive days (total dose 42 g/m2) and subsequent autologous PBPCT. To date we have transplanted 3 patients, all female. Two patients (#1, #3) had symptomatic splenomegaly and severe anemia. Patient #2 had symptomatic splenomegaly and thrombocytopenia (< 100/nl). The WB nadir after treosulfan therapy was 0.06/nl (#1), 0.28/nl (#2) and 0.09/nl (#3). The time to reconstitution of leucocytes > 1/nl post transplantation was 28 days (#1, #2) and 38 days (#3) and the time for reconstitution of thrombocytes > 50/nl was 36 (#1), 22 (#2) and 33 (#3) days. The prolonged reconstitution period may have been due to the myelofibrosis and has also been observed after busulfan conditioning and PBPCT by others. There were no fever or other severe toxicity. Patients did not require total parenteral nutrition. The patients have been followed-up for 18 months post transplantation, now. The first patient (#1), who required erythrocyte transfusions twice weekly pretransplant received her last erythrocyte transfusion on day 56; her Hb-value is 11.2 g/dl at last follow-up. The second patient (#2) recovered to platelet counts higher than pre-transplantation (58/nl) at 3 months.(143/nl) and had 103/nl at last follow-up. Pat. #3 showed a marked reduction of max. spleen size and a rise in Hb from 9 g/dl to 12 g/dl after 12 months, the Hb value at 18 months is 8.7 g/dl and allogeneic transplantation is considered now. We conclude that myeloablation with treosulfan and autologous PBPCT in these three patients with myelofibrosis was safe and useful. Survival of these patients is promising up to date. The results presented warrant further exploration of this approach in a clinical study which has been initiated by our group. (cf. http://leukaemie.krebsinfo.de/kn_home/Studien/studie_101.html).