Clinically latent myeloproliferative disorders (MPDs) are important causes of what would otherwise be considered idiopathic hepatic (HVT) or portal vein thrombosis (PVT). They may be difficult to diagnose initially because the peripheral blood count may be normal at the time of thrombosis. A strong association between an activating mutation of the gene encoding one of the Janus kinase family of tyrosine kinases (JAK2(V617F)) and the Philadelphia chromosome-negative MPDs has been identified. We have studied 19 patients with unexplained HVT or PVT and tested for JAK2(V617F). Fourteen (74%) of the 19 patients were heterozygous for JAK2(V617F) but did not meet diagnostic criteria for a MPD at the time of presentation with thrombosis. Prolonged follow-up established the presence of an overt MPD in 13 of the 14 patients after a median duration of 38 months. We recommend testing for JAK2(V617F) in all patients with unexplained HVT or PVT, to identify latent MPDs and prevent potential complications.