Objective:To assess the influence of congenital and permanent unilateral hearing loss (UHL) on early prelingual auditory development (EPLAD) of infants and toddlers.Study Design:A cross-sectional and case-controlled design.Setting:Tertiary referral center.Patients:Sixty-five young children (median with interquartile ranges: 4.4 [3.3, 7.7] mo) with UHL, 70 children (6.0 [3.8, 9.0] mo) with bilateral normal hearing (BNH) and 32 children (5.8 [3.9, 12.1] mo) with bilateral mild hearing loss (BMHL) were enrolled and grouped. Children with UHL were further grouped according to the severity of impairment and the ear that was impaired.Interventions:Children involved were assessed by the 9-item Infant-toddler Meaningful Auditory Integration Scale (ITMAIS).Main Outcome Measures:1) EPLAD assessed by ITMAIS, 2) functions of sound detection, sound discrimination and identification and preverbal vocalization assessed by dividing ITMAIS into three sections, 3) trajectories of EPLAD estimated by ITMAIS.Results:Compared with BNH, children with UHL lagged behind in the function of sound discrimination and identification, preverbal vocalization, as well as trajectory of EPLAD. Children with severe to profound UHL exhibited more severe EPLAD handicaps than children with mild to moderate losses. The auditory functions and EPLAD trajectory of children with UHL were similar to those with BMHL.Conclusions:Children with congenital and permanent UHL demonstrate delays in EPLAD, which are evident soon after birth before initial development of speech and language. The ITMAIS is an appropriate tool to identify these delays both in children with unilateral and bilateral hearing impairment.