The body cavities are rarely involved in hairy cell leukaemia. Here we report a patient who had pancytopenia, hepatosplenomegaly, massive haemorrhagic ascites, pleural effusion at the left hemithorax and increased CA 125 serum level at the time of initial diagnosis. Laparoscopy showed multiple nodular white, opaque lesions on the omentum and on the parietal peritoneum. Laparoscopic biopsy of these lesions, and a bone marrow biopsy revealed a diffuse cellular infiltrate of tartrate-resistant acid phosphatase staining mononuclear cells. These mononuclear cells with irregular cytoplasmic protrusions were also found in the peripheral blood, in the ascites fluid and in the pleural effusion. The patient was treated with cladribine 0.1 mg/kg/day with continuous infusion for seven days. Three months after the treatment, the patient achieved a complete remission with normalisation of the peripheral blood count, bone marrow findings, CA 125 serum level, with no detectable ascites and/or pleural effusion.