Recent reports suggest that human immunodeficiency virus (HIV)- associated progressive multifocal leukoencephalopathy (PML) may improve with highly active antiretroviral therapy (HAART). We observed three patients who developed PML while receiving HAART. All patients received HAART for 4-11 months and had low plasma levels of HIV-1 RNA before the onset of symptoms of PML. Antiretroviral therapy was changed in two patients, and their plasma HIV-1 RNA levels declined significantly. Despite this virologic response, PML did not improve in these patients. The third patient's HIV-1 RNA level became undetectable while he was receiving HAAT, and his symptoms of PML improved after the addition of interferon α. Our observations suggest that PML can develop in patients who have shown clinical response to HAART. Furthermore, PML may not improve despite an adequate virologic response to HAART. Definitive therapy is still needed for PML.