This study evaluated the efficacy and tolerability of budesonide in an aqueous nasal spray (BANS) in patients with chronic rhinosinusitis. In this double-blind, placebo-controlled, multicentre, parallel-group study, patients (n = 167) with persistent rhinosinusitis symptoms despite 2-weeks' antibiotic treatment were randomised to receive BANS 128 micrograms b.i.d. or placebo for 20 weeks. Morning combined symptom scores (CSS) in patients receiving BANS decreased by a mean of -1.85 (95% CI -2.27, -1.43), versus -1.02 (-1.43, -0.61) in the placebo group (p = 0.005); corresponding values for evening CSS were -1.78 (-2.22, -1.35) and -1.02 (-1.45, -0.60), respectively (p = 0.012). BANS produced significant reductions in nasal congestion and discharge scores, and improved patients' sense of smell (morning only), versus placebo. Peak nasal inspiratory flow (PNIF) increased significantly during BANS treatment. In allergic patients, BANS significantly (p < 0.001) reduced both morning -1.40 (-2.18, -0.62) and evening -1.37 (-2.15, -0.58) CSS from baseline versus placebo, but changes in non-allergic patients (morning: -0.04 [-0.95, 0.87]; evening: 0.14 [-0.81, 1.09]) were not significant. PNIF was significantly (p < 0.01) increased in both allergic and non-allergic patients from baseline versus placebo. BANS is an effective and well-tolerated treatment for chronic rhinosinusitis.