Hairy-cell leukaemia (HCL) is a low grade B-cell lymphoproliferative process that presents either with lymphocytosis or neutropenia/monocytopenia. It is a disease predominantly of bone marrow and spleen, although it can also involve other organs and sites. Recent advances including multi-parameter flow cytometry and the development of antibodies with high specificity for HCL have permitted precise distinction of typical HCL from other lymphoproliferative diseases that can morphologically mimic the appearance of HCL. Although there is a commonly held belief that HCL is associated with a significant increase in second neoplasms, several recent studies have not supported this notion. The development of extremely effective therapy for HCL results in a high incidence of complete remission. However, a significant percentage of patients continue to harbour minimal residual disease that can be revealed with immunohistochemical and flow cytometric studies.