To analyze the clinical efficacy of cidofovir combined with highly active anti-retroviral therapy (HAART) in AIDS-related progressive multifocal leukoencepalopathy (PML), a multicenter observational study was performed. Consecutive HIV-positive patients with histologically or virologically proven PML and at least 4 weeks of treatment after diagnosis were examined: 27 patients were treated with HAART, whereas 16 patients were treated with HAART plus cidofovir 5 mg/kg intravenously per week for the first 2 weeks and every other week thereafter. JC virus DNA was quantified in cerebrospinal fluid (CSF) by PCR. Baseline virologic, immunologic, and clinical characteristics as well as HIV RNA and CD4 responses to HAART were homogeneous between the groups. The median follow-up was 132 weeks. In one case (6%), cidofovir was permanently discontinued because of severe proteinuria. One-year cumulative probability of survival was 0.61 with cidofovir and 0.29 without (log rank test P = 0.02). After adjusting for baseline CD4 counts, JC viral load in CSF, Karnofsky, and use of HAART prior to the onset of PML, the use of cidofovir was independently associated with a reduced risk of death (hazard ratio, 0.21, 95% confidence interval, 0.07-0.65; P = 0.005). A randomized study will definitively establish whether cidofovir confers significant advantage over HAART alone in AIDS-related PML.