OBJECTIVES/HYPOTHESIS: Acute rhinosinusitis is a frequent common cold-related complication in children. Despite the need for appropriate treatment, its underlying microbiology remains unclear. This study aimed to investigate the microbiology of acute rhinosinusitis in children. STUDY DESIGN: Prospective non controlled study. METHODS: Thirty-one pediatric acute maxillary sinusitis patients with severe symptoms were assessed. The subjects were 17 males and 14 females aged 5 to 14 years (mean age, 9.1 years). Maxillary sinus aspirates were collected and cultured, with subsequent viral and bacterial polymerase chain reaction (PCR) analysis. Bacteria were analyzed using culturing and PCR, and viruses were analyzed using PCR. The PCR kits used identify 18 types of respiratory viruses and 13 types of bacteria. RESULTS: At least one pathogen was detected in 30 of 31 aspirates (97%) using PCR, and none of the aspirates contained respiratory viruses alone. Ten aspirates (32%) contained both viruses and bacteria. The most common viruses detected were rhinovirus (13%) and influenza virus (10%). The most common bacteria were Haemophilus influenzae (45%), Streptococcus pneumoniae (32%), Moraxella catarrhalis (16%), and Chlamydophila pneumoniae (13%). Bacteria were found in 21 of 31 cases (68%) via bacterial culturing. Culturing revealed that H influenzae was the most common pathogen (42%). CONCLUSIONS: In pediatric acute maxillary sinusitis, respiratory bacteria were detected in 65% of the sinus aspirates and both bacteria and viruses in 32%. The most common viruses were rhinovirus and influenza virus, and the most common bacteria were H influenzae and S pneumoniae. Viral and bacterial PCR is useful for accurately investigating the microbiology in pediatric sinusitis. LEVEL OF EVIDENCE: 3 Laryngoscope, 131:E2705-E2711, 2021.