Objective/Hypothesis: Several inflammatory and allergic disorders have been reported to correlate with interleukin-16 (IL-16). IL-16 is a highly potent chemotactic and chemoattractant molecule for eosinophils. Nasal polyposis (NP) has an inflammatory basis, and eosinophilia is a common finding in polyp tissue. In this study, we aimed to determine serum IL-16 levels in NP patients. Study Design: Prospective controlled study in a tertiary academic hospital. Methods: Peripheral blood samples were obtained and used for serologic studies with enzyme-linked immunosorbent assay kits to determine IL-16 levels. Results: The study and the control groups consisted of 17 and 10 patients, respectively. Mean serum IL-16 levels were significantly higher in the study group when compared to the control group (447.9 pg/mL vs. 260.2 pg/mL) (P <.001). Conclusions: Serum IL-16 levels are significantly elevated in NP patients. This finding may represent identification of a valuable serum biomarker and potential therapeutic target in NP patients. Copyright © 2012 The American Laryngological, Rhinological, and Otological Society, Inc.