Recently, an activating somatic mutation of Janus kinase 2 (JAK2V617F) was identified in the myeloproliferative disorders (MPDs). In this study, we investigated the occurrence of JAK2V617F in patients with slightly elevated platelets or hemoglobin without a secondary cause, who did not meet the criteria of polycythemia vera or essential thrombocythemia. Six out of 18 patients (33%) were positive for the JAK2 mutation, and five of these six patients had a history of thrombosis. These findings suggest that apart from thrombocytosis/erythrocytosis, other mechanisms exist that cause thrombosis, and more patients with a latent form of MPD are likely to exist. Future studies will have to elucidate how to treat these patients.