Objective: To study the development French as a mainstream language by children with a cochlear implant who belong to non-French speaking families. Study Design: Matched pairs comparison of postoperative hearing perception and speech development data of monolingual and bilingual children with cochlear implants. Setting: University medical center. Patients: Fourteen congenital profoundly deaf children. Seven were exclusively French speaking and seven bilingual (French-Portuguese, Arab, Turkish, or Serbo-Croatian) children. Interventions: Cochlear implantation before the age of 52years. Main Outcome Measures: Subjects were evaluated using standard hearing perception and oral language development tests. Results: Both monolingual and bilingual groups obtained excellent hearing perception results with the cochlear implant. Monolingual children showed oral language development results equivalent to those of normal hearing children of the same age, except for the morpho-syntax test where they were slightly below average. Bilingual children scored below average in all oral language development tests. Conclusion: Despite the excellent hearing perception obtained with cochlear implants, the acquisition of a second language at home seems to slow down the development of the French mainstream language. Comparison of our results with those of previous studies indicates that bilingual children require intensive and correct input in both languages. Parents' involvement in rehabilitation efforts also appears as an important factor for successful oral language development.