Background: Unilateral, mucopurulent drainage from an isolated paranasal sinus may be encountered in patients with a history of surgery for the treatment of chronic rhinosinusitis (CRS). Static mucus is visualized on nasal endoscopy within the sinus lumen but without significant disease in the adjacent sinuses. The reasons for this phenomenon are unknown although an iatrogenic cause is proposed.; Methods: A case series was prospectively compiled from consecutive patients presenting for evaluation of CRS at a tertiary rhinology practice during a 16-month period. Computerized tomography and nasal endoscopy were performed, and endoscopically directed aerobic and anaerobic bacterial cultures were obtained. Osteitis scores were recorded for diseased and nondiseased sides.; Results: Twenty-three of 113 patients (20.4%) had evidence of chronic unilateral drainage from either a maxillary (21) or sphenoid (2) sinus. Mean osteitis scores were higher for the diseased side (P < 0.01). A nonendoscopic transantral approach was reported in 57.1% of cases with chronic maxillary disease, with 52.2% occurring more than 10 years earlier. The most common bacterial isolate was Pseudomonas aeruginosa (6 cases, 26.1%), followed by methicillin-resistant Staphylococcus aureus (5 cases, 21.7%). Six cases (26.1%) were polymicrobial, and 6 (26.1%) were culture-negative. Tobacco use was reported in 8 (34.8%) cases, and chronic obstructive pulmonary disease was present in 6 (26.1%) cases.; Conclusion: The condemned sinus is a distinct entity that may represent a sequela of previous non-mucosal-sparing surgery. An association with hyperostosis is observed. Mucopurulent drainage is characterized by polymicrobial infection comparable to that found in diffuse CRS.