This trial includes patients with chronic lymphocytic leukemia, the most common kind of malignant lymphoma. Monoclonal antibodies directed against cluster of differentiation antigen 20 have improved treatment results in different forms of lymphomas; however in chronic lymphocytic leukemia treatment with monoclonal antibodies is less effective, and it has been suggested that this is depending on a lower expression of the cluster of differentiation antigen 20 protein on the chronic lymphocytic leukemia cells.

Valproate, an anticonvulsant drug, has been shown to increase the cluster of differentiation antigen 20 expression, and the rationale in this study is that an increasement of cluster of differentiation antigen 20 would make treatment with monoclonal antibodies in patients with chronic lymphocytic leukemia more effective.

Inclusion Criteria:

• Age 18-80 years
• Histologically confirmed chronic lymphocytic leukemia
• Leucocyte count more than 20 x 10 9/L
• No other simultaneous treatment for lymphoma
• No treatment indicated for chronic lymphocytic leukemia
• WHO performance status 0-2
• HIV negativity
• Negativity for hepatitis C virus , HBsAG, anti-hepatitis B core antigen, or other active infection uncontrolled by treatment
• Agree to abstain from donating blood while taking study drug therapy and for one week following discontinuation of study drug therapy
• Agree not to share study medication with another person and to return all unused study drug to the investigator
• Written informed concent

Exclusion Criteria:

• Psychiatric illness or condition which could interfere with the subjects' ability to understand the requirements of the study
• Neurological or neuropsychiatric disorder, interfering with the requirements of the study
• Hearing impairment over grade 2
• Porphyria
• History of acute or chronic hepatitis
• Family history of severe drug-induced hepatitis
• Pregnancy and lactation