The trial will test the effectiveness and toxicity of subcutaneous treatment with one cycle of cladribine in patients with hairy cell leukemia requiring treatment.

They have to be untreated so far or may be pretreated with alpha-interferon.

Evaluation of remission status will take place 4 months after treatment. In addition, it will be tested whether patients with non-optimal response will have a benefit from a second cycle of cladribine.

Non-optimal response is: patients with detectable residual disease; achievement of partial remission or detectable residual infiltration in the bone marrow.

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