Twenty-five patients with a primary myelodysplastic syndrome (MDS) transformed into acute non-lymphoblastic leukaemia (ANL) were treated with intensive chemotherapy. A complete remission (CR) was obtained in six patients (24 per cent). In five of these six patients two courses of chemotherapy were needed to achieve CR. In eight patients chemotherapy cleared the bone marrow of blasts, but the aplasia was fatal. A partial effect on bone marrow blasts was seen in four patients and no effect in another six. Eleven patients (44 per cent) died from the consequences of chemotherapy-induced cytopenia. A short interval between MDS and transformation into ANL was associated with a better chance of achieving complete remission. Age, karyotype, type of MDS, peripheral blood or bone marrow findings had no influence on the result of chemotherapy. The median survival from start of treatment was 5 months (range 0.5-24 months). In the patients who achieved a CR, the median duration of the remission was 7 months (range 3-12 months). The poor response rate, the short duration of the remissions and the high treatment-related mortality suggest that current intensive anti-leukemic chemotherapy in ANL after primary MDS is of limited benefit.