Myeloproliferative diseases represent a major risk factor for Budd-Chiari syndrome. In 32 patients with Budd-Chiari syndrome, the JAK2 V617F mutation was detected, in heterozygous state, in 11 individuals (34.4%; 95% confidence interval: 18.6-53.2). Eight patients with (72.7%; 95% confidence interval: 39.0-94.0) and six without (28.6%; 95% confidence interval: 11.3-52.2) the JAK2 V617F mutation had a diagnosis of myeloproliferative diseases before or at the occurrence of the venous thrombotic event. In three patients carrying the JAK2 V617F mutation, a myeloproliferative disease was not detected. Determination of the JAK2 V617F mutation may be useful to recognize patients with Budd-Chiari syndrome with or at risk for the subsequent development of overt myeloproliferative diseases.