OBJECTIVE: To investigate the formation of bacterial biofilms on the surface of the electrode array of cochlear implants (CI) explanted because of device failure, without evidence of infection, by use of scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM). STUDY DESIGN: Prospective study. SETTING: Patients from 2 tertiary-care referral centers. PATIENTS AND METHODS: CIs were explanted from 9 patients because of device failure. Specimens were immediately snap-frozen in cold isopenthane, stored at -80°C and examined with SEM and CLSM by 3 investigators. MAIN OUTCOME MEASURE: Presence of bacterial biofilm ascertained by SEM and CSLM. RESULTS: One specimen showed the formation of a bacterial biofilm on the middle ear part of the electrode array. No biofilm formation was found in the inner-ear part of electrode arrays. In the middle-ear part of the electrode array, a cylindrical cover of human muscular tissue was seen plugging the cochleostomy. CONCLUSION: This is the first study demonstrating that bacterial biofilms may exist on the surface of the electrode array of CIs explanted because of device failure but not infection. We found 1 case of biofilm formation in 9 explanted CIs. Further studies with larger series of CIs are required to investigate biofilm formation on the surface of CI electrode arrays to address both the pathophysiology of bacterial biofilms and prevention of device-related infections in CI patients.