The apparent selectivity of agnosia for faces is termed prosopagnosia or face blindness. This cognitive dysfunction can be seen after traumatic events--involving at least the right occipital temporal region--or very frequently congenital in the absence of any detectable lesions. The familiarity of congenital prosopagnosia was studied in two independently ascertained collections of subjects with prosopagnosia. One was an unselected group of pupils and students who underwent a questionnaire based screening. The others were self reported subjects after having heard for the first time about the phenomenon of prosopagnosia from mass media citing our studies and/or from our homepage (www.prosopagnosia.de). Those who agreed with consecutive studies of their family members had mostly one or more prosopagnosic first degree relatives. The segregation patterns derived from 39 families are compatible with autosomal dominant inheritance. Hence, mutation(s) in one gene are sufficient for manifestation of the phenotype. Still fitting the concept of autosomal dominant inheritance, we have evidence for a slightly reduced penetrance (4 normal transmitters from distinct families) and one or two de novo mutations.