Chronic rhinosinusitis (CRS) is one of the most common chronic diseases. Its etiology is unknown, and there is a paucity of effective medical treatments. We tested the hypothesis that intranasal antifungal treatment improves the objective computed tomography (CT) findings (inflammatory mucosal thickening), nasal endoscopy stages, and symptoms of CRS. A randomized, placebo-controlled, double-blind, single-center trial used amphotericin B to treat 30 randomly selected patients with CRS. Patients were instructed to instill 20 mL amphotericin B (250 mug/mL) or placebo to each nostril twice daily for 6 months. The primary outcome was a quantitative reduction in inflammatory mucosal thickening on CT scans of a standardized coronal cut. Secondary outcome measures were endoscopic scores, patient symptom scores, and levels of intranasal inflammatory mediators. Twenty-four patients completed the 6 months of treatment. Patients receiving amphotericin B achieved a relative reduction in the percentage of mucosal thickening on CT scans (n = 10; -8.8%) compared with placebo (n = 14; +2.5%; P = .030). Likewise, the changes in the endoscopic scores improved in the amphotericin B group compared with placebo ( P = .038). Between-group comparisons of the changes in the intranasal mucus levels of eosinophil-derived neurotoxin showed a reduction in the amphotericin B group and an increase in the placebo group ( P = .046); levels of IL-5 showed similar tendencies ( P = .082). Intranasal amphotericin B reduced inflammatory mucosal thickening on both CT scan and nasal endoscopy and decreased the levels of intranasal markers for eosinophilic inflammation in patients with CRS.