Functional endoscopic sinus surgery (FESS) is being advocated to treat children with chronic sinusitis. The surgeon performing FESS needs an intimate understanding of the anatomy to adequately treat disease and avoid complications. One hundred thirty-six patients who had endoscopic sinus surgery were reviewed. Preoperative direct coronal computed tomography scans were available for review in 114 patients. Several nasal and paranasal sinus anomalies were uncovered during this evaluation. The most common was hypoplasia of the maxillary sinuses. All hypoplastic maxillary sinuses had diseased mucosa. Eleven patients were found to have a laterally deviated uncinate process and abnormal anatomy of the maxillary ostia. Other anomalies such as concha bullosa, Haller's cells, and paradoxic curvature of the middle turbinate were found, and their respective frequencies in this population were determined.