We describe the clinical and virological outcome of human immunodeficiency virus-infected patients with progressive multifocal leukoencephalopathy (PML) treated with cytarabine. Twenty-seven patients received intrathecal cytarabine, 5 received concomitant intravenous cytarabine, and 20 received concomitant antiretroviral therapy. The median baseline CD4+ cell count was 28/mm3. After 4 weeks, 4 (19%) of 21 evaluable patients had stable disease, whereas the others progressed. The median survival from diagnosis and from onset was 66 and 128 days, respectively. Patients with Karnofsky scores of >50 and those previously taking antiretroviral medications had a higher probability of survival 3 months after diagnosis (P = .003 and P = .05, respectively). Overall, after 4 weeks, median JC virus load in CSF increased by 0.7 log10 copies/mL from baseline (P = NS). The mean JC virus load at 4 weeks was lower in patients with stable disease than in progressors (3.47 vs. 4.47 log10 copies/mL; P = .027). JC virus became undetectable in the only patient who had a long-term stable condition. The concentration of JC virus in CSF showed a correlation with clinical outcome.