Inclusion Criteria:

- Acute lymphoblastic leukemia in first or subsequent complete remission

- Indication for allogeneic stem cell transplantation according to the actual protocol
of the German Acute Lymphoblastic Leukemia Study Group

- Patient's age: 18-65 years

- HLA-identical or compatible related or unrelated donor (HLA-A, HLA-B, HLA-C, HLA-DRB1
and HLA-DQB1) (one antigen-mismatch allowed)

- Not eligible for total-body irradiation due to one of the following reasons:

- prior radiation of the spine > 30 Gy

- prior radiation of the mediastinum > 30 Gy

- severe pulmonary infection during induction chemotherapy

- DLCO > 50%

- Patient's wishing to avoid total-body irradiation as conditioning regimen

- Patient's written informed consent

- Women and men capable of reproduction must agree to use highly effective methods of
contraception until six months after treatment termination. For men: vasectomy,
sexual abstinence, or partner is using hormonal IUD, implants, injectables, oral
hormonal contraceptives or is surgically sterilized. For women: hormonal IUD,
implants, injectables, sexual abstinence, surgical sterilization, vasectomised
partner

Exclusion Criteria:

- No complete remission at time of registration

- Severe irreversible renal, hepatic, pulmonary or cardiac disease, such as

- total bilirubin, SGPT or SGOT > 3 times upper the normal level

- Left ventricular ejection fraction < 30%

- Creatinine clearance < 30 ml/min

- DLCO < 35% and/ or receiving supplementary continuous oxygen

- Positive serology HIV

- Pregnant or lactating women

- Severe florid infection

- Experienced hypersensitivity against cyclophosphamid, etoposide, or treosulfan

- Cystitis

- Obstructive renal function

- Participation in any other clinical drug trial

- Serious psychiatric or psychological disorders

- Progressive invasive fungal infection at time of registration