We investigated language functions in 32 members of a four generation family with several members affected by Specific Language Impairment with an extensive language test battery in order to determine the prevalence, overlap, and homogeneity of linguistic deficits within one pedigree. In sum, one fourth of all family members tested fulfilled the criteria of Specific Language Impairment. Despite of some similarities in language abilities, different combinations of language deficits were observed, and individual language profiles varied substantially. Thus, though there is a high prevalence of language deficits in this family which raises the likelihood of a genetic origin of these deficits, and though all affected study participants displayed selective linguistic deficits with normal non-verbal functioning, language testing showed considerable variance in overlap and homogeneity of linguistic deficits. Thus, even in one genetic population, an underlying linguistic disorder manifests itself in different language abilities to a variant degree.