The clinical course and causative organisms were studied in 18 patients with acute mastoiditis, 13 of whom (72%) had no previous history of middle ear disease. Their age ranged from 5 months to 21 years, and duration of middle ear symptoms immediately prior to admission ranged from 1 to 45 days (average 9.7 days). None had undergone a myringotomy prior to admission, while 13 (72%) had been receiving antibiotic treatment for acute otitis media. Three were admitted with intracranial complications. Bacteria were isolated in 10 of the 16 patients in whom samples were available for bacterial culture, and included Streptococcus pneumonia (2), Streptococcus pyogenes (2), Staphylococcus aureus (2), Staphylococcus coagulase negative (2), Klebsiella pneumonia (1), and Pseudomonas aeruginosa (1). Of the 17 patients treated by us, 11 received surgery. Acute otitis media, secretory otitis media, acute mastoiditis, subacute mastoiditis and masked mastoiditis create a continuum. Antibiotic treatment for acute otitis media cannot be considered as an absolute safeguard against acute mastoiditis. When antibiotics are prescribed for acute mastoiditis before culture result is available, an anti-staphylococcal agent should be included. At least some patients with acute mastoiditis develop a primary infection of the bony framework of the middle ear cleft. The prevalence of the intracranial complications in acute mastoiditis is still high and may appear soon after or concomitant with the first sign of acute mastoiditis.