OBJECTIVE:To evaluate and compare clinical and microbiological findings in adults hospitalized for acute otitis media (AOM) or mastoid infections (acute or latent). MATERIALS and METHODS:We retrospectively reviewed the medical records of all adults (≥17 years old) hospitalized (between 2003 and 2012) at a tertiary referral center for acute mastoid infections or AOM not responding to outpatient medical treatment. RESULTS:Of the 160 patients in the study sample, 19% had an infection caused by S. pyogenes, 14% by S. pneumoniae, and 11% by P. aeruginosa. AOM was the most common infection (38%), whereas 33% had acute mastoiditis (AM), 18% had latent mastoiditis (LM), and 13% AM of a chronically infected ear (AMc). In contrast to the other infections, P. aeruginosa (30%) and S. aureus (25%) were most common in AMc. Otorrhea (83%), tympanic membrane perforation (57%), and hearing problems (83%) were common in S. pyogenes infections. Patients with S. pneumoniae had longer lengths of hospitalization than those with other bacterial infections (7 vs. 4 days). Otorrhea (94%) and retroauricular symptoms were more common in P. aeruginosa infections. Hearing symptoms were common (67%) but fever (32%) and retroauricular symptoms were uncommon in AOM. Fever (44%) and mastoid tenderness (65%) were common in AM. Patients with LM underwent the most mastoidectomies (54%). Prior medical conditions, retroauricular symptoms, otorrhea (90%), and post-infection problems were common in AMc. CONCLUSION:The bacteriological etiology of hospitalized AOM more closely resembled those of LM and AM than that of AMc. Adults hospitalized for AOM or AM required fewer mastoidectomies than those hospitalized for LM or AMc.