BACKGROUND Pure alexia, or alexia without agraphia, is usually caused by disconnection between the gyrus angularis and visual areas of both hemispheres. Cerebrovascular disease is the most frequent cause of this syndrome. METHODS We report the first case of a patient with pure alexia secondary to progressive multifocal leukoencephalopathy (PML) as the first manifestation of Human Immunodeficiency Virus (HIV) infection. Polymerase chain reaction in cerebrospinal fluid was positive for JC virus. Despite favorable immunological and virological response to highly active antiretroviral therapy, neurological symptoms and demyelinating lesions progressed during the first months, and right homonyous hemianopsia, acalculia, and color anomia were noted. In magnetic resonance imaging (MRI) demyelinating lesions involved white matter of both occipital lobes, with left predominance, forceps major and splenium of the corpus callosum, together with white matter of the left temporal lobe and both hippocampal regions. During the following months, coinciding cidofovir treatment, the patient had partial clinical improvement, but neurological deficits persisted. The size of MRI lesions significantly improved. CONCLUSIONS PML is frequently the first manifestation of AIDS (acquired immunodeficiency syndrome). It should be considered as a possible diagnosis of pure alexia.