Interferon alpha-2b (αIF) was administered to 13 previously untreated patients with essential thrombocythaemia (ET). Pretreatment median platelet count was 1.178 (range, 662 to 1,700) x 10(9)/L, with ten patients showing values above 1,000 × 10(9)/L. Six patients had vascular symptoms attributable to ET, whereas in the remaining cases treatment was instituted due to either an increased vascular risk or platelet counts higher than 1,000 × 10(9)/L. With an induction regimen of αIF 3 MU/day, given subcutaneously, a rapid decrease in the platelet counts was observed in all cases. Two patients did not complete the induction therapy because of unacceptable toxic side-effects at the time their platelet counts approached normal values. In the remaining 11 patients a complete haematologic response (platelets below 400 × 10(9)/L) was observed, after a median of 9.5 (range, 2.6 to 36) weeks from the start of therapy. In only 3 patients was dose escalation of interferon to 5 MU/day required. Eleven patients experienced flu-like symptoms, that could be well controlled by oral paracetamol in 7 cases. After discontinuing therapy a rise in the platelet counts was observed in all patients, after a median of 5 (range, 2 to 35) weeks. Two patients subsequently refused to continue on interferon therapy, whereas in the remainder a dose of 3 MU 2 to 4 times weekly maintained normal platelet counts.