Acute mastoiditis is an acute inflammation of the mastoid process with bone erosion. It is a complication of acute otitis media, which is rare but with increasing incidence. Distinct characteristics are an erythema and oedematous swelling of the skin of the mastoid process. A fluctuant swelling points to a subperiosteal abscess. Laboratory examination and imaging only support the diagnostics. Therapy involves obligatory systemic antibiotic treatment. At the beginning of the inflammation a paracentesis can be sufficient. A mastoidectomy must be carried out if clear signs of an osseous necrolysis, such as a subperiosteal abscess, are present. The most frequent causative agents are gram positive cocci. Intraoperative smears are best suitable for microbiological diagnostics. Complications of acute mastoiditis are encroachments of the inflammation on neighbouring structures of the mastoid. In such cases a tomography is indicated and therapy is usually surgical.