To determine the relationship between prior antibiotic use and the development of acute mastoiditis (AM) in children. Retrospective review. Tertiary pediatric center. We identified 129 patients with AM who were admitted to our center between 1996 and 2005. Proportion of children who developed AM. A total of only 67 patients (51.9%) had undergone any antimicrobial treatment prior to hospital admission. In 1996, 64% of patients with AM had received antibiotics for acute otitis media (AOM) prior to admission (n = 7 of 11), but this percentage had steadily decreased to 27% by 2005 (n = 4 of 15). The yearly number of cases of AM treated in our institution has remained stable over this period. A subperiosteal abscess was identified in 45 patients (34.9%), while the remainder (n = 84) had postauricular inflammation only (65.1%). Nineteen patients with a subperiosteal abscess (42%) and 48 patients without a subperiosteal abscess (57%) had undergone prehospitalization antimicrobial therapy for suppurative AOM. There was no significant difference in antibiotic use between the numbers of patients with or without a subperiosteal abscess. Regarding the sensitivity of bacteria isolated from patients with a subperiosteal abscess, only 1 patient was infected with an organism that was not sensitive to the prehospitalization antibiotic prescribed. Use of antibiotics to treat suppurative AOM in children might not influence the subsequent development of AM.