A 39-year-old intravenous drug user presented with dysarthria and a syndrome of the left cerebellar hemisphere. While in hospital, he developed progressive brainstem findings. Repeated CT scans revealed a lucency in the white matter of the left cerebellar hemisphere. Brainstem auditory and short-latency somatosensory evoked potentials provided evidence of brainstem dysfunction without corresponding lesions on CT. Biopsy of the cerebellum established the diagnosis of progressive multifocal leukoencephalopathy (PML) and the acquired immunodeficiency syndrome (AIDS). Postmortem examination revealed brainstem lesions appropriate to the evoked potential findings and a radiographically inapparent lesion in the right internal capsule. Based on this case and a review of the literature we conclude that: (1) PML occurs with significant prevalence in AIDS patients and may involve the posterior fossa; (2) the diagnosis of posterior fossa PML is suggested by certain clinical and radiographic criteria and may be confirmed by brain biopsy; (3) evoked potentials may be abnormal in PML and can reveal functional abnormalities of white matter without apparent CT abnormalities.