MRI was used to study the effects of introducing cidofovir (HPMPC, Vistide) to the antiretroviral therapy of a 33-year-old white man diagnosed as having progressive multifocal leukoencephalopathy (PML) secondary to AIDS. In response to combined cidofovir and antiretroviral therapy he showed significant clinical improvement. MRI showed a decrease in extent of existing lesions, without new ones. Blood chemistry information obtained indicated some involvement of immunologic mechanisms: the CD4:8 ratio showed improvement from an average of 0.08 before treatment to 0.13 during therapy.