Ruxolitinib is a novel therapeutic agent which is being used in the treatment of myelofibrosis. In the literature, there are reports of opportunistic infections along with tuberculosis during ruxolitinib treatment. Herein, we aimed to report a post-polycythemic myelofibrosis case that developed mycobacterial infection after ruxolitinib treatment. 64 years old man was diagnosed as polycythemia vera 27 years ago. He was diagnosed as post-polycythemic myelofibrosis in 2002. He was given hydroxycarbamide and interferon treatments, however ruxolitinib treatment was started in July 2015 because of spleen enlargement and constitutional symptoms. Ruxolitinib was started with a dose of 2x5 mg and the dose gradually increased to 2x20 mg by September 2015. His constitutional symptoms decreased and cachexia improved by January 2016. However he started to have fever attacks. Investigations revealed no origin for fever. The quantiferon test was resulted positive in May 2016. Tuberculosis PCR test was negative. However, ADA level was found to be increased in cerebrospinal fluid. General condition of the patient deteriorated rapidly and he was lost in June 2016. Afterwards "mycobacterium tuberculosis complex" was detected in our patient's culture sample. Ruxolitinib has serious effects on immune system. To conclude, patients who are given ruxolitinib should be closely followed-up especially in the first months of treatment and benefits and prophylactic use of antivirals and antibiotics should be investigated in future studies.