Therapeutic leukapheresis was performed in a patient with hairy cell leukaemia (100,000 WBC/mm3; 98 p. 100 hairy cells) without signs of hypersplenism. Twenty-five sessions were carried out in 12 weeks. A rapid and significant improvement was observed with the WBC falling to 25,000 WBC/mm3 with 70 p. 100 of hairy cells and the normalisation of the bone marrow biopsy. When these sessions were finished, splenectomy was performed in optimal conditions. This treatment would seem to be valuable in hairy cell leukaemia without severe hypersplenism. Long-term improvement may be hoped because hairy cells have a very short turnover rate.