Hyperhomocysteinemia is a risk for thrombosis. Thrombosis is more frequent in myeloproliferative neoplasms (MPNs) than the normal population. In this study, we investigated the thrombosis incidence, the effects of plasma homocysteine levels on thrombosis, and the correlations between folate, cobalamin, pyridoxine and homocysteine levels in MPNs, and to compare these results with the healthy controls.