Thirty-six patients with chronic myeloproliferative disorders (CMPD) with thrombocytosis (essential thrombocythaemia 19 patients, chronic megakaryocytic granulocytic myelosis five, polycythaemia vera six, chronic myelogenous leukaemia six) were treated with interferon alfa-2b to reduce the platelet count. The pre-treatment platelet count was in the range 450-700 x 10(9)/L in eight patients, 700-1000 x 10(9)/L in eight and above 1000 x 10(9)/L in 20. In the induction phase of treatment 22 patients were treated with interferon alfa-2b 3 million units (MU) daily subcutaneously for 2 months or until the platelet count returned to normal, if earlier. Fourteen patients received 5 MU interferon alfa-2b daily in the same way. In the maintenance phase the doses were reduced to 3 MU and 5 MU thrice weekly, respectively. Complete response (CR), defined as a reduction of platelet count to below 450 x 10(9)/L, was achieved in 78% of patients (within 2 months of induction in 64%). The platelet depleting effect was dose dependent: CR in 2 months in 52% on 3 MU interferon alfa-2b versus 75% on 5 MU. Reduction of interferon dose was followed by an increase in platelet count in 39% of patients. The white cell count fell by 50% in Philadelphia-negative CMPD. Side effects were common, though generally mild, but led to withdrawal of treatment in six patients. Three patients suffered cerebrovascular events during treatment and one shortly thereafter.