- Background. Acute mastoiditis is a serious complication of acute otitis media (AOM) and has been increasingly reported in the last decade. Objectives. To report the experience accumulated with acute mastoiditis at the Soroka University Medical Center, Beer-Sheva, Israel, in a period of increasing antimicrobial resistance with Streptococcus pneumoniae. Patients and methods. We reviewed the records of all children with acute mastoiditis hospitalized from 1990 through 2001. Acute mastoiditis was diagnosed when one or more of the physical signs of mastoiditis (swelling, erythema, tenderness of the retroauricular area and anteroinferior displacement of the auricle) were diagnosed in the presence of concomitant or recent (≤4 weeks) AOM. Results. One hundred sixteen episodes of acute mastoiditis occurred in 101 children age 2 months to 14 years (median, 25 months; 19% <1 year old). The average yearly incidence was 6.1 cases per 100 000 population <14 years old, with a significant increase in the number of cases during the study period. Acute mastoiditis was the first evidence of AOM in 10 (10%) patients. Fever >38°C and >15 000 WBC/mm3 were present in 67 and 43% of cases, respectively. Irritability, retroauricular swelling, redness and protrusion of the auricle occurred more commonly in patients <3 years old (79, 90, 84 and 76%vs. 28, 42, 45 and 30%, respectively, in patients ≥3 years old; P < 0.002). Computed tomography scans were performed in 54 of 116 (47%) cases and revealed bone destruction in 38 (70%). Periosteal abscess and lateral sinus vein thrombosis were diagnosed in 8 and 2 patients, respectively. Simple mastoidectomy was done in 32 of 116 (28%) cases, after no response to intravenous antibiotics was observed; ventilation tubes were inserted in 12 patients. Cultures were obtained at admission in 83 (72%) episodes (71 by tympanocentesis and 12 from ear discharges). Overall 43 pathogens were isolated: 34 at admission; 14 at surgery; and 5 at both occasions. The most commonly isolated organisms were S. pneumoniae (14 of 43, 33%; 4 penicillin-nonsusceptible), Streptococcus pyogenes (11 of 43, 26%), nontypable Haemophilus influenzae (6 of 43, 14%), Pseudomonas aeruginosa (5 of 43, 12%) and Escherichia coli (4 of 43, 9%). All S. pneumoniae organisms were isolated between 1996 and 2001. Conclusions. (1) The incidence of acute mastoiditis in children in Southern Israel is greater than that reported in the literature; (2) a significant increase in the number of cases was recorded during the study period; (3) the clinical picture of acute mastoiditis was more severe in infants and young children; (4) the pathogen distribution in acute mastoiditis differs from that of AOM with significantly higher rates of S. pyogenes and lower rates of nontypable H. influenzae recovery; (5) penicillin-nonsusceptible S. pneumoniae played only a minor role in the etiology of acute mastoiditis in Southern Israel.