The association of V617F JAK2 expression levels with disease behavior has not been studied in patients with nonchronic myelogenous leukemia (CML) myeloproliferative disease (MPD). We found plasma levels of total JAK2 mRNA to be higher in patients with non-CML MPD (n = 175) than in CML patients (n = 45) and normal controls (n = 58) (each P < 0.001). Overall survival was studied in 68 patients and showed positive correlation with levels of total and mutant JAK2 mRNA in patients with the V617F mutation, but not those without the mutation. These findings suggest that total JAK2 expression levels play a role in the biology of the disease in V617F-positive patients, and a therapy aiming at downmodulating the expression of the total JAK2 mRNA should be considered. In conclusion, we studied JAK2 total and V6217F mutant mRNA levels in plasma. We show high levels of JAK2 expression in MPD patients and these levels correlate with survival.