Hairy cell leukaemia (HCL) is a chronic progressive disease of predominantly middle-aged men. Alpha interferon has been shown to induce significant responses in HCL patients. With interferon treatment the platelet count normalizes first, followed by the haemoglobin and neutrophil counts. The number of hairy cells in the bone marrow decreases and granulocytic, erythroid and megakaryocytic cells increase. Interferon is well tolerated with the most common side effect being a flu-like syndrome. A number of HCL patients will develop neutralizing antibodies and in these cases the chemotherapeutic agents pentostatin and 2-chlorodeoxyadenosine should be considered. Preliminary results with these agents are promising and further trials are ongoing to confirm their clinical promise.