To evaluate the effects of treatment with ruxolitinib (INCB018424) on spleen volume, symptoms and potential side effects in patients with PMF, PPV-MF and PET-MF who have platelet counts of 50 x 10^9/L to 100 x 10^9/L. It is anticipated that individualized dose optimization from the starting ruxolitinib level of 5 mg bid will be associated with reductions in splenomegaly, MF-associated symptoms and inflammatory cytokine levels.