MRI, 1H-MR spectroscopy, immunologic, and virologic studies were performed in search of prognostic factors of disease evolution in patients with progressive multifocal leukoencephalopathy (PML). Acute lesions of PML survivors showed twofold higher standard score of the ratio of myoinositol (a glial marker) to creatine compared with lesions in patients whose disease progressed. Concomitantly, JC virus-specific cytotoxic T lymphocytes were only detected in the blood of PML survivors. These results suggest that inflammation limits disease progression.