Inclusion Criteria:

• Patients with histologically verified hairy cell leukemia
• Presence of hairy cells in the bone marrow and peripheral blood detected by positive TRAP staining and / or co expression if cell surface antigens cluster of differentiation (CD) 19/CD25 or CD19/CD103 (b-ly7)
• No previous cytostatic treatment (splenectomy or interferon treatment are allowed)
• Need for treatment
• Age at least 18 years old
• General state of health according to WHO 0-2
• Current histology, not older than 6 months
• Written consent by patient

Exclusion Criteria:

• Patients not fulfilling inclusion criteria above
• Hairy cell leukemia variants (HCL-V): presence of lymphoid cells in bone marrow and / or peripheral blood, which have an intermediate morphology between hairy cells and prolymphocytes (negative TRAP staining and co- expression of CD19/CD103 without CD25
• Pretreatment with purine analogues or other chemotherapeutics
• Concomitant corticosteroid therapy
• Severe dysfunction of the heart (NYHA III or IV), the lung (WHO-Grade III or IV), the liver, except due to lymphoma (bilirubin > 2 mg/dl, alkaline phosphatase, glutamate-oxalacetate transaminase and glutamate-pyruvate transaminase > 2 x upper limit of normal), the kidneys (creatinin > 2 mg/dl or creatinine clearance < 50 ml/min), central nervous system diseases including psychoses.
• Proven HIV infection
• Active Hepatitis
• Other florid infections
• Anamnesis / diagnosis of other malignant disease (other than non-melanoma associated skin tumours or stage 0 in situ carcinoma of the cervix)
• Pregnant or lactating women