A young man without antecedent episodes of rhinosinusitis or sinus surgery presented with progressive headache and left periorbital edema and pain recalcitrant to 4 days of outpatient oral antibiotics and 2 days of intravenous vancomycin and ceftriaxone. His family history was positive for a similar presentation involving his mother. Physical examination revealed left orbital swelling and erythema with proptosis. Cranial nerve function and ocular range of motion were intact. Contrast-enhanced computed tomographic (Figure, A) and magnetic resonance imaging studies of the paranasal sinuses (Figure, B) disclosed extensive preseptal and postseptal orbital inflammation, bilateral frontal sinus opacification from a large left frontal osteoma occupying the inferior aspect of the left frontal sinus, and left epidural abscess. The patient underwent a bicoronal approach osteoplastic flap and endoscopic frontal sinusotomy for access and drainage of left frontal sinus disease and obstructive osteoma excision. In addition to frontal sinus osteoma, multiple 3- to 5-mm osteomas covering the calvarium were encountered (Figure, C). The patient’s postoperative period progressed without incident, and he was discharged home with oral antibiotics following resolution of symptoms. Histopathologic findings of operative specimens showed dense lamellar bone (Figure, D). Genetic analysis was positive for mutation in the APC gene at 5q21, resulting in a stop codon.