To explore the correlations among the incidence, microbiology, season, gender, and age in patients with peritonsillar abscess (PTA) in order to identify risk factors for PTA development. All patients with PTA treated at private ENT practices in Aarhus County and in the Ear-Nose-Throat Departments at Aarhus University Hospital and Randers Hospital from January 2001 to December 2006 were included in the study. Age- and gender-stratified population data for Aarhus County for the same 6 years were obtained. The incidence rate of PTA increased from childhood to peak in teenage life and declined afterward gradually until old age. Girls predominated over boys until the age of 14 years. Subsequently, men were more frequently affected than women. Fusobacterium necrophorum (FN) was significantly more prevalent than group A Streptococcus (GAS) among patients aged 15-24 years (P < 0.001). In contrast, GAS was significantly more frequently recovered among children aged 0-9 years and adults aged 30-39 years compared with FN (P < 0.001 and P = 0.017 respectively). The seasonal variation of PTA was statistically insignificant (P = 0.437). However, GAS was significantly more frequently recovered in the winter and spring than in the summer (P = 0.002 and P = 0.036 respectively). There was a trend toward a higher incidence of FN infection during the summer than the winter (P = 0.165). Although the collected PTA incidence was stable throughout the year, the microbiology fluctuated with seasons. Patients aged 15-24 years are at an increased risk of PTA due to FN, which may have clinical implications for the diagnostic work-up and treatment of patients with acute tonsillitis.