Ninety ET patients diagnosed according to the PVSG criteria entered a phase II study designed to identify a treatment with PEG Interferon α-2b (PEG Intron, Schering-Plough Corp) safe and able to obtain and to maintain for a long time the Hematological Response (HR, PLTs < 500x 109/L). The patients, 30 Males and 60 Females, median age 45 years (18–72), judged at risk and in the majority of cases pre-treated with cytoreductive drugs (60%), at study entry showed a mean PLT count of 1093 ± 357 x 109/L (>1000 in 49% of cases). During the first year (part 1 of the study) the PEG Intron starting dose of 25 μg/week was gradually increased if necessary at week 13, 26 and 39 till a maximum of 100 μg/week to reach the HR. The HR at the end of the 1st year (primary endpoint) was registered in 64 patients who represented the 71% of the 90 enrolled subjects (analysis on Intent-to-Treat, ITT) and the 79% of the 81 patients still receiving PEG Intron. The PEG Intron effective dose had a mean value of 50 ± 22 μg/week resulting very low (25μg/week) in 33% of cases. The primary endpoint was reached independently by sex, age, cytoreductive pre-treatment and baseline PLT count. Nevertheless, the HR was more frequent in the patients with true ET respect those with false ET (WHO criteria, HR 93% vs. 68%). In the 64 responder patients admitted to the part 2 of the study according to the protocol, the HR at the end of the 2nd year was registered in 75% of cases on ITT and in 87% of the 55 patients still on treatment, with a PEG Intron maintenance dose gradually reduced to a mean level of 27 μg/week. No thrombotic or hemorrhagic events occurred during the study and, moreover, a decrease of splenomegaly (from 22% to 6%) and of disease related symptoms (from 42% to 2%) was registered. The unmaintained HR, in 29 valuable patients who discontinued the PEG Intron treatment after two years, had a median duration of 11 weeks (4–52). Eighteen patients discontinued PEG Intron treatment (9 during the 1st year and 9 during the 2nd year) as consequence of PEG Intron toxicity and side effects (n 7), loss of compliance (n 5) and drug unrelated reasons (n 6). Toxicity and side effects (WHO grading) were never of grade 4, in very few cases of grade 3 and in the majority of patients of grade 1 or 2 (mainly flu-like syndrome, leukopenia, hypertransaminasemia). The ECOG Performance status (PS) was of grade 0 in all but few patients who showed a transitory grade 1 PS. In conclusion, PEG Intron treatment performed at relatively low dose seems safe and able to induce and to maintain the HR for a long time in the majority of at risk ET patients.