Thirteen patients (mean age 60.7 years; female:male ratio 10:3) with essential thrombocythaemia were treated with 3 million units (MU)/day interferon alfa-2b subcutaneously (s.c.) for 12 weeks, with all patients requiring a dose reduction after 4 weeks. The mean pretreatment platelet count was 1,400 x 10(9)/L and megakaryocytes were increased in all cases. Splenomegaly was present in six patients and haemorrhagic phenomena were observed in two. Nine patients (69.2%) had objective responses, including two (15.4%) complete responses (platelets less than 450 x 10(9)/L) which were then maintained with 5 MU interferon twice a week. Acute toxicity consisted of flu-like symptoms in 12 patients. Chronic toxicity (mainly leucopenia) was observed in nine patients. In conclusion, initial therapy and then requiring maintenance therapy at a reduced dose. However, the frequent side effects observed make it advisable to use a low dose of interferon alfa-2b, and to treat only those patients with significant symptoms and signs of thrombocytosis.