Previous family history studies have demonstrated that there are elevated rates of language and language-related impairments in families identified through probands with language impairments. This study examines family histories of children with specific language impairment (SLI) known to have particular grammatical limitations in a core feature of grammatical acquisition, a stage known as Extended Optional Infinitives (EOI). Family affectedness rates are reported for 31 families identified through preschool probands with this clearly defined language impairment and 67 control families, identified through nonaffected preschool children developmentally similar to the probands. It was found that significantly more speech and language difficulties, as well as language-related difficulties, such as reading, were reported for proband families than control families. The elevated rates were obtained for nuclear family members and extended family members as well. Fathers of probands were more often reported as having difficulties (29% for speech/language impairments) than were mothers of probands (7%), but there was no difference between brothers (26%) and sisters (29%). No differences were evident between proband families based on proband gender. The findings are relevant for theoretical models of sources of unexplained variations in grammatical competence in young children. In addition, the findings contribute new information about expected rates of affectedness, means of identification of affected family members, and comorbidity of symptomatology.