This open label study determined the outcome of non-AIDS progressive multifocal leukoencephalopathy patients treated with a standard dose of intravenous cytosine arabinoside. Nineteen patients with PML proven by brain biopsy or spinal fluid polymerase chain reaction were treated with intravenous cytosine arabinoside 2 mg/kg per day for 5 days and followed for neurologic outcome by neurologic examination and MRI scanning. Seven of 19 PML patients treated with cytosine arabinoside intravenously improved neurologically. The range of follow-up for these patients was 2.0 to 4.5 years. All were left with neurologic deficits but were functionally improved, and 6 of 7 were able to independently carry out the activities of daily living. Twelve PML patients showed no evidence of response and died rapidly of their disease after treatment (range, 8 days to 6 months). All who survived their neurologic disease recovered from treatment-induced pancytopenia. Cytosine arabinoside given intravenously to non-AIDS PML patients in this small study was associated with a 36% chance of developing stabilization at 1 year. Treatment was associated with significant bone marrow toxicity. The improvement in MRI scan changes in those patients who responded took 6 weeks or longer.