- Summary Acute mastoiditis (AM) is an uncommon but serious complication of acute otitis media (AOM). In the pre-antibiotic era, AM was seen more frequently than it is today, but it was rare in infants. However, in the last two decades an increase in the incidence of AM in infancy has been reported in the literature. During the years 1990–2002, we treated 113 patients with 128 episodes of AM; of them, 24 were infants (median age 6 months; 18 males) who suffered from 26 episodes of AM. Twenty developed AM as a complication of their first episode of AOM. One of the four infants with a prior history of AOM suffered from common variable immunodeficiency. A significant increase in the incidence of AM in infants was recorded during the study period ( P = 0.01). The most common presenting clinical signs were post-auricular swelling and fever >38 °C (77% and 77%, respectively, of all patients). Seventeen episodes of AM were not treated with prior antibiotics. Tympanocentesis was performed in all episodes of AM. Middle ear fluid culture was positive in 17 (65%) of the 26 AM episodes. The most common pathogens cultured were Streptococcus pneumoniae (10 infants, 58% of all pathogens, 3/10 intermediately susceptible to penicillin) followed by Streptococcus pyogenes (4, 23%), non-typable H. influenzae (2, 12%) and S. aureus (1, 6%). Temporal bone CT showed bone destruction in 14 patients; 3 infants had subperiosteal abscesses and 3 lateral sinus thrombosis. Ten infants underwent mastoid surgery due to non-resolution of symptoms and signs with antibiotic therapy. Eight underwent cortical mastoidectomy with two patients undergoing ventilation tube introduction only. The remainder of the infants healed with conservative treatment. Conclusions : (1) A significant increase in the incidence of AM in infants was recorded over the last decade, though a specific reason for this trend remains uncertain; (2) Most of the cases of AM followed the infant's initial AOM episode, and most of the infants had not received prior antibiotic therapy; (3) The clinical signs and symptoms of AM were more severe in infants than in older patients; (4) While S. pneumonia was the most common pathogen isolated in middle ear fluid cultures, the involvement of S. pyogenes in AM was higher than that reported in AOM.