We report a case of progressive multifocal leukoencephalopathy (PML) isolated to the posterior fossa in a 55-year-old male with acquired immune deficiency syndrome(AIDS). Initial MR images revealed a few foci of patchy increased signal intensity(SI)on a T2-weighted image and a diffusion weighted image (DWI) at the pons, right middle cerebellar peduncle, and right cerebellar hemisphere, with no enhancement. After anti-retroviral therapy, follow-up MR images revealed the more prominent extent of previously-seen lesions and newly discovered newly developed focal increased SI on T2-weighted images located left of the inferior cerebellar hemisphere. Proton MR spectroscopy (1H-MRS) showed a slightly increased choline peak (3.2 ppm) and lactate peak (1.35 ppm), as well as a decreased N-acetylaspartate (NAA) peak (2.0 ppm), which suggests active demyelinating disease. DWI and 1H-MRS may support the diagnosis of PML in patients with AIDS.