Interferons are new and effective agents in the treatment of various haematological neoplasias. Alpha-interferon (natural or recombinant) has a high efficacy (90% response rate) in hairy cell leukaemia. Complete remissions are, however, rare and definite cure of the disease is unlikely. Alpha-interferon induces haematological remissions in about two thirds of patients with chronic myeloid leukaemia and leads to a reduction in Philadelphia chromosome in about 40% of patients. It is uncertain, however, whether this treatment will actually prolong the life of these patients as compared with conventional treatment. Alpha-interferon has a beneficial effect in some patients with low malignant non-Hodgkin lymphomas (in particular follicular lymphomas). The response rate in myeloma is rather small (20%). Gamma-interferon is not effective in hairy cell leukaemia, non-Hodgkin lymphoma and myeloma. It is, however, of some efficacy in chronic myeloid leukaemia (the response rate in lower than with alpha-interferon) and possibly has some effect in patients with acute myeloid leukaemia and myelodysplastic syndromes. The toxicity of interferons (alpha and gamma) consists of an influenza-like syndrome during the first days of treatment. Low doses of alpha-interferon show virtually no long-term toxicity. However, bone and muscular pain is sometimes dose-limiting with intermediate doses (5 to 15 million units) of alpha-interferon.