A case story is presented, describing a 46 y old man, with a relapsing hairy cell leukaemia. After treatment with monoclonal anti CD-20 antibodies (rituximab) 375 mg/week, four times, a complete remission was obtained which has lasted >9 months. The rituximab treatment produced a better remission than earlier treatments with alpha-interferon and chlorodeoxyadenosine. In addition, in contrast to other treatments, no initial worsening of the pancytopenia was observed.