OBJECTIVE: To describe the progression of sensorineural hearing loss (SNHL) in the better- and poorer-hearing ears in children with asymptomatic congenital cytomegalovirus (CMV) infection with isolated SNHL. STUDY DESIGN: Longitudinal prospective cohort study. SETTING: Tertiary medical center. SUBJECTS AND METHODS: We analyzed hearing thresholds of the better- and poorer-hearing ears of 16 CMV-infected patients with isolated congenital/early-onset or delayed-onset SNHL identified through hospital-based CMV screening of >30,000 newborns from 1982 to 1992. RESULTS: By 12 months of age, 4 of 7 patients with congenital/early-onset SNHL developed worsening thresholds in the poorer-hearing ear, and 1 had an improvement in the better-hearing ear. By 18 years of age, all 7 patients had worsening thresholds in the poorer-hearing ear and 3 patients had worsening thresholds in the better-hearing ear. Hearing loss first worsened at a mean age of 2 and 6 years in the poorer- and better-hearing ears, respectively. Nine patients were diagnosed with delayed-onset SNHL (mean age of 9 years vs 12 years for the poorer- and better-hearing ears), 6 of whom had worsening thresholds in the poorer-hearing ear and 1 in both ears. CONCLUSION: In most children with congenital CMV infection and isolated SNHL, the poorer-hearing ear worsened earlier and more precipitously than the better-hearing ear. This study suggests that monitoring individual hearing thresholds in both ears is important for appropriate interventions and future evaluation of efficacy of antiviral treatment.