OBJECTIVES: We previously presented evidence for the existence of a "disproportionate language impairment" (DLI) in a small sample of children using a cochlear implant (CI) who had a good surgical outcome and who were otherwise unimpaired, including typical nonverbal intelligence quotient. Herein, we studied a larger sample of children with DLI who were CI users and, as before, paired them with closely matched, "Control" children who were CI users but had more typical language skills. The primary aim was to assess the communication skills of siblings of each group of CI users to seek evidence for the inheritance of language and more general communicative impairments in the DLI group. We also investigated laterality as a possible coindicator of impairment among the CI users. DESIGN: Prelingually deaf children using a single CI and presenting with severe unexplained language problems (DLI-CI, N = 25) were compared with a group of children using a CI who did not present with such difficulties (Control-CI, N = 25). The groups were matched on age, gender, etiology, age at implantation, implant manufacturer, duration of deafness, and CI experience. Members of both groups completed a series of nonverbal and verbal tests used to identify specific language impairment in normal-hearing (NH) children, and tests of cerebral laterality. Parents completed the Children's Communication Checklist (CCC-2) for each of their children (i.e., the CI users and all their siblings; sibling results were grouped as DLI-Sibling and Control-Sibling) and a questionnaire regarding family history of hearing, speech, language, and literacy problems of the children's first-degree relatives. RESULTS: Significant differences of performance on all language tests were found between the DLI-CI and the Control-CI groups. No difference was found between the groups on performance intelligence quotient or auditory memory. CCC-2 results indicated lower General Communication Composite and higher Social Interaction Deviance Composite scores in the DLI-CI than in the Control-CI group. All children in the DLI-CI group produced abnormal overall profiles compared with less than half the children in the Control-CI group. CCC-2 results for the siblings showed a significant association between sibling group and communication; 35% in the DLI-Sibling group produced abnormal profiles compared with 8% in the Control-Sibling group. Co-occurring deafness and language problems in a small number of siblings and other family members did not account fully for these communication difficulties, but there was a higher prevalence of these difficulties in the families of both groups than is reported in NH families. Laterality tasks did not show significant differences between the groups of CI users, although there were trends for children in the DLI-CI group to have less dominant hand preference and eye-hand dominance than those in the Control-CI group and in NH children. CONCLUSIONS: Language tests, including the CCC-2, could identify communication difficulties in children using CIs, leading to prompt identification, informed intervention strategies, and managed expectations. Evidence from their siblings suggests that these difficulties may not all be determined by their deafness or device characteristics but through the same heritable and environmental factors that influence language development in all children.