A 42-year-old man with chronic myelogenous leukemia presented to the hospital with hip pain. After undergoing surgical repair for a hip fracture, he developed aphasia, facial droop and fever. He was initially diagnosed with a stroke. A lumbar puncture excluded common infections, and an ELISA test for HIV was negative. Serial MRI scans revealed a progressive demyelinating pattern. Polymerase chain reaction performed on cerebrospinal fluid was positive for the JC virus (JC are the initials of the first person diagnosed with this virus). The patient was treated with cidofovir but died before a full course could be administered. JC virus is present in most adult hosts but is only harmful in immuno-compromised hosts. The active form of the disease is known as progressive multifocal leukoencephalopathy. This is the first reported case of a patient developing progressive multifocal leukoencephalopathy. with chronic myelogenous leukemia as the only underlying condition.