The primary thrombocythemia (PT)-1 trial (NCT00175838) comprises a series of three studies, which have been running since 1997 and include very large cohorts. PT, also known as essential thrombocythemia (ET), is a chronic disease, so long-term studies like this are essential. Speaking from American Society of Hematology (ASH) 2017 Annual Meeting and Exposition in Atlanta, GA, Claire Harrison, MD, DM, FRCP, FRCPath, of Guy’s and St Thomas’ NHS Foundation Trust, London, UK, discusses the results of one of the arms of the study, which was presented at the meeting. This arm explored the pre-emptive addition of hydroxycarbamide (hydroxyurea) to aspirin in intermediate risk ET, investigating whether this reduced the risk of vascular events, or myelofibrotic or leukemic transformation. Prof. Harrison highlights how the results of this will influence patient treatment moving forwards.