A 41-year-old male homosexual with AIDS was hospitalized for temperature elevation to 40 degrees C with confusion. Neurologic evaluation found psychomotor slowing and temporospatial disorientation with no focal signs. The CD4 count was 100/mm3. CSF analysis and the CT scan were normal. Despite antiviral treatment the patient died fifteen days after admission. Gross appearance of the brain was normal. Histologic examination disclosed multiple, small foci of demyelination characteristic of progressive multifocal leukoencephalopathy. These foci were disseminated among the U fibers. In situ hybridization and immunocytochemical studies demonstrated papovavirus particles in oligodendrocytes and a few astrocytes. This case shows that papovavirus infection in AIDS patients may be responsible for a diffuse febrile encephalopathy with normal CT scan findings and a rapidly progressive course.