EDITOR,I read with interest the excellent review on posterior leukoencephalopathy syndrome published in January.¹ The author, however, has omitted an important differential diagnosis in his articlenamely, progressive multifocal leukoencephalopathy (PML). This can mimic the appearances of posterior leukoencephalopathy on both computed tomography and magnetic resonance imaging (MRI) scans of the brain and needs to be high on the differential diagnosis especially in patients with AIDS.² PML was first described in 1958³ and is characterised by widespread demyelination in the cerebral hemispheres. Today PML is seen most frequently in patients with AIDS but can also occur in patients with chronic neoplasia and immunosuppressed states. Intellectual changes, hemiparesis, visual field defects, ataxia, aphasia, and dementia are clinical features. Seizures are rare. The cerebrospinal fluid is usually normal. On computed tomography there may be low attenuation areas in the posterior fossae but MRI (T2 weighted) shows characteristic increased signal . . . [Full text of this article]