Objective The objective of this multicentre retrospective case review was to assess the natural clinical course, efficacy and safety of Mastoid Obliteration with S53P4 Bioactive Glass (bioactive glass). Study Design Retrospective case note review Setting Regional Tertiary Referral centre and District General Hospital Participants Patients undergoing mastoid cavity obliteration as part of primary or secondary procedure with bioactive Glass between 2012 – 2018. Outcome measures were assessed from a prospectively collated database and case note review. Main Outcome Measures Primary outcomes were the common morbidities of a mastoid cavity; dry or discharging ear (Merchant’s scale), vertigo in cold air and a water tight middle ear. Patients were also assessed for audiological outcomes and recidivism. Results 90 patients were included. During the follow up period, (mean = 22 months; range = 6 - 59 months) cholesteatoma recidivism was observed in 2% of ears, (2 patients). An acceptably dry (Merchant Grade 0 – 1) ear was achieved in 91% of all ears (95% Primary Cases, 80% Secondary Cases). Delayed healing of the graft in the external ear canal retaining the S53P4BAG Bioactive Glass (BonAlive,(BonAlive Biomaterials Ltd., Turku, Finland)) within the mastoid occurred in 13% (12 ears). However, in all cases conservative management resulted in complete healing. Conclusions Bioactive glass provides a safe and effective means of mastoid obliteration. Complications including overlay graft failure and slow epithelisation resulting in prolonged postoperative discharge (up to 2 months) and dehiscence into the external ear canal do not preclude full recovery and may be successfully managed conservatively.