Catastrophic intra-abdominal thrombosis can result from a variety of prothrombotic states, including polycythemia vera and essential thrombocythemia, both of which are frequently associated with an acquired mutation (V617F) in the JAK2 gene. To assess the prevalence and clinical implications of this mutation in the setting of intra-abdominal thrombosis, JAK2 V617F genotyping was performed in 42 patients who had catastrophic intra-abdominal thromboses resulting in visceral transplants. The prevalence of V617F was compared with that of other prothrombotic states for which molecular testing is routinely performed. V617F mutations were detected in 7 patients (17%), who were not distinguishable on the basis of their peripheral blood cell counts. The median posttransplantation survival of V617F+ patients was 17.5 months, compared with 116.4 months for the V617F- patients (ratio, 6.6; 95% confidence interval, 6.3-7.0). These results highlight the diagnostic usefulness of JAK2 V617F testing in this setting and underscore the clinical significance of a positive result.