This report presents the results of a clinical trial on 53 patients with hairy cell leukaemia using low-dose alpha-interferon as therapy. Improvement of cytopenia and/or bone marrow hairy cell infiltration occurred in all but one patient. Often, blood and bone marrow improvements were dissociated and, after 7 or 13 months of therapy, complete remission was only observed in about 40% of patients. Recurrence of the disease was observed in some cases after cessation of therapy. A summary of the following results is presented: alpha-interferon receptor analysis, oncogene expression, study of the sensitivity of hairy cells to natural killer cells, and the effects of interferon on T-cell receptor gene rearrangement and on the function of T-cell clones. Results are also presented which show that myelofibrosis may be due to a release of platelet derived growth factor. The immunological findings and oncogene expression in 2 patients with a variant form of hairy cell leukaemia for which resistance to therapy was observed are also described. All the results show that interferon acts on hairy cells and are consistent with a direct effect by interferon in the treatment of hairy cell leukaemia.