We investigated single-word reading in normal subjects and patients with alexia following a left occipital infarct, using PET. The most posterior brain region to show a lateralized response was at the left occipitotemporal junction, in the inferior temporal gyrus. This region was activated when normal subjects, patients with hemianopic alexia and patients with an incomplete right homonymous hemianopia, but no reading deficit, viewed single words presented at increasing rates. This same area was damaged in a patient with pure alexia ("alexia without agraphia") and no hemianopia, who read words slowly using a letter-by-letter strategy. Although the exact level of the functional deficit is controversial, pure alexia is the result of an inability to map a percept of all the letters in a familiar letter string on to the mental representation of the whole word form. However, the commonest deficit associated with "pure" alexia is a right homonymous field defect; an impairment that may, by itself, interfere with single-word reading because of inability to see the letters towards the end of a word. The relative contributions of pure and hemianopic alexia in individual patients needs to be assessed, as the latter has been shown to respond well to specific rehabilitation programmes.