To clarify the relationship between the levels of JAK2 wild-type (WT) and V617F mutant-positive platelets in patients with essential thrombocythaemia (ET), we quantified mutant levels in purified cells from 10 V617F-positive patients prior to receiving cytoreductive therapy. Mutant levels were significantly higher in platelet than neutrophil RNA (P = 0.002), but the mutation was still only present in a sub-population of platelets (median 54%). When the absolute number of WT platelets was calculated, it was always within or above the normal platelet range, indicating that there is an aberration in the negative feedback to JAK2 WT platelets in ET.