Since the discovery of the JAK2V617F mutation, the clinical and pathological consequences of this acquired defect have been extensively investigated to determine whether its presence characterises a distinct subgroup of myeloproliferative disorders (MPD). MPD management remains highly dependent on the patient’s thrombotic risk. Whether the presence of the JAK2V617F mutation modifies the thrombotic risk is currently contentious, although there is increasing clinical evidence to suggest that the mutation may be variably associated with thrombosis. These observations are further supported by laboratory parameters which suggest that the JAK2V617F mutation may confer increased activation of leucocytes and platelets in MPD. The role of screening for the JAK2V617F mutation in patients presenting with thrombosis without overt MPD is unclear, but appears justified in cases of idiopathic splanchnic vein thrombosis.