Hairy-cell leukaemia (HCL) differs from all other haematological malignancies in two major respects. First, the disease is characterized by a range of highly unusual and distinctive biological features, all of which probably reflect the activated nature of hairy cells (HCs), the pathognomonic malignant cells of the disease. Secondly, the disease is unusually sensitive to therapy withunconventional'agents (interferon-a (IFN-a) and nucleosides). Intuitively, one feels that these two unusual features of HCL must be related, but the nature of any such relationship is as yet unclear. Here we review both the biology and management of HCL, and very briefly speculate how the unusual biological features of the disease may be related to its high level of sensitivity to treatment.

Biology HCL is a chronic B-cell leukaemia with a variable propensity for blood involvement and marrow suppression. As in other chronic lymphoid malignancies, the abnormal cells have a low proliferative rate and their accumulation is therefore primarily the result of abnormally prolonged cell survival. Although the nature of the underlying oncogenic event (s) is unknown, it results in the distinctive activated phenotype of HCs and in specific patterns of tissue involvement; both of these features are likely to be relevant to the prolonged survival of the malignant cells. The distinctive biological features of the disease are summarized in Table I. We will deal with each of these in turn and then consider some factors which potentially may be responsible for the activated nature of the HC.