This study investigated the neural basis and cognitive mechanisms of disorders affecting the visual processing of human faces. Two patients affected by prosopagnosia (PA), 45 unilateral brain-damaged patients not previously selected for gnosic disturbance, and 52 normal subjects were administered three tasks involving visual matching of unfamiliar faces, identification of famous faces, and age attribution to unknown faces. Results confirmed that the right hemisphere (and particularly the posterior region) is critical for all of the main aspects of facial processing. Neuroradiological and PET examination in both PA patients revealed a lesion in the territory of the right posterior cerebral artery (infero-mesial areas of temporal and occipital lobes). Moreover, group comparisons showed that patients affected by a right posterior lesion obtained the worst scores on all tasks of our battery. Both group comparisons and single-case analyses pointed out that a deficient recognition of familiar faces is generally independent of perceptual disorders and, as such, should be considered mnestic in nature. Disorders in the age attribution, on the contrary, appear to be strictly associated with deficits in the perceptual analysis of unfamiliar faces.