Twenty-nine late chronic and accelerated phase chronic myelogenous leukaemia (CML) patients were entered in a pilot study designed to test the therapeutic efficacy of treatment with interferon-alpha (IFN-alpha) and low-dose cytosine arabinoside (ARA-C). IFN-alpha was administered at a dose of 2-10 x 10(6) IU/day and ARA-C at 15 mg/m2/day for 14 days each month. The treatment was well tolerated by 73% of the patients. Side effects were mainly asthenia, anorexia, anaemia and piastrinopenia. Haematological and cytogenetic responses were evaluated in the 19 patients who received more than 6 cycles. Four complete haematological response, 7 partial haematological response, 6 minor haematological response, 2 stable disease were obtained in this patient group. Two complete cytogenetic responses and 2 minor cytogenetic responses were detected in these patients. Suppression of secondary Ph' positive clones which appeared during the previous IFN-alpha treatment was documented in 3 accelerated phase patients after ARA-C was added to their IFN-alpha treatment. It would therefore seem that late chronic and accelerated phase CML patients benefit from combined IFN-alpha/ARA-C treatment and achieve haematological and cytogenetic responses not obtained during previous treatment without being exposed to undue toxicity. However, we cannot judge whether it offers any advantage in terms of survival.