In a neuropathological autopsy study of 21 cases of AIDS-associated PML no fundamental morphological differences to non-AIDS PML were found. PML in AIDS often showed very large foci as well as necrotizing lesions. Partial involvement of cerebral cortex and deep gray matter were common findings; infratentorial lesions could be observed in more than three quarters of cases. Perivascular mononuclear infiltrates within PML foci were frequent and obviously not associated with a more benign clinical course. Possible reasons for these peculiarities of PML in AIDS are discussed. In 7 cases evidence of concomitant HIV encephalopathy was found; this may be one relevant factor contributing to severity of PML in AIDS. PML has to be regarded as a common complication of HIV infection, which may show atypical morphological and neuroradiological features.