The development of children who have syndromes with mental retardation and/or language delay can be worsened by sensorineural or conductive hearing loss (HL). Given the existing scarcity of data, we investigated the prevalence of otitis media with effusion and/or HL in 50 children with Cornelia de Lange syndrome (CdLS) aged 1-18 years, and its impact on the children's performance. The children underwent otological and audiological examinations in order to ascertain the relative frequencies of otitis media with effusion and/or hearing impairment; their demographic and clinical data were obtained by questionnaires and from information in their medical charts. Otitis media with effusion was diagnosed in 94%, and its prevalence was similar in all age groups; HL was detected in 40 children (80%). Conductive HL due to middle ear effusion was the main cause of hearing impairment alone (60%) or in combination with sensorineural deficit (20%). HL had a negative impact on performance regardless of the type. A history of routine audiological and/or otological assessments was reported by a minority of parents. Our findings indicate that otitis media with effusion and/or HL is an important feature of children with CdLS and may negatively affect their performance. Careful follow-up throughout childhood is necessary to detect and treat any hearing loss in children with CdLS in order to minimize its impact on performance.