15 autopsy cases of PML in AIDS are presented. Frequent findings were: Very large, confluent PML-lesions (7 cases), lesions with prominent necrotic features (10 cases) and with marked perivascular mononuclear infiltrates (6 cases). In 9 cases additional PML-foci were seen in the cerebellum and/or the brain stem, and in 1 case there was even an exclusive involvement of those structures. Such characteristics regarding severity and topography of PML in AIDS, may be due to additional toxic factors. In 7 cases HIV-encephalopathy with multinucleated macrophages was present.