Objective: To assess long-term results and present experience with a high-porosity hydroxyapatite ceramic for obliterating large open mastoid cavities. Study-Design: Cross-sectional cohort study. Setting: Tertiary academic referral center. Patients: All patients who underwent tympanomastoid surgery for chronic middle ear disease or revision surgery with reduction of an open mastoid cavity using a highly porous hydroxyapatite matrix material (HMM) between May 2005 and June 2013 were assessed for eligibility. Twenty three patients (56.9 ± 18.3 yr) were included. Intervention: Primary middle ear surgery or revision surgery using a HMM. Main Outcome Measures: Pure-tone average, computed tomography (CT), and magnetic resonance imaging (MRI) to investigate osseoinduction, osseointegration and presence of cholesteatoma, current quality of life assessed by Zurich Chronic Middle Ear Inventory and change in quality of life post-intervention assessed by the Glasgow Benefit Inventory. Results: Patients were reexamined after a mean follow-up period of 88.3 months (SD 21.4 mo) after obliteration of the open mastoid cavity with HMM. Compared with visit 1, patients showed a significantly reduced ABG at visit 2 (29.22 dB ± 2.71 dB versus 12.77 dB ± 3.46 dB). CT scan was carried out in 21 patients (91%) patients and 17 patients (74%) underwent MRI. Revision surgery was required in a total of 17 cases (74%). In four patients recurrent cholesteatoma was found at follow up. Conclusions: Poor cavity obliteration, a high rate of revision surgery and difficult differentiation between recurrent cholesteatoma and granulation tissue in CT scan was observed.