A randomized, double-blind, placebo-controlled trial was performed to assess the efficacy of once daily budesonide in patients with nasal polyps. After a 2-week run-in period, 157 patients with symptomatic bilateral nasal polyposis were randomized to receive budesonide, 140 micrograms once or twice daily or 280 micrograms once daily (delivered doses) via Turbuhaler, or placebo for 8 weeks. Polyp size was assessed endoscopically and, in two centres, by magnetic resonance imaging (MRI). Nasal symptoms (blocked nose, runny nose, sneezing) were recorded daily, and patients provided an overall assessment of efficacy at the end of the study. Budesonide, 280 micrograms/day (280 micrograms o.d. and 140 micrograms twice daily), significantly reduced polyp size, compared with placebo, whereas budesonide, 140 micrograms once daily, had no significant effect. Nasal polyp mass score, measured by MRI, was also significantly reduced in patients receiving 280 micrograms/day. All three doses of budesonide significantly reduced symptom scores, and there were no significant differences between the groups. Overall, approximately 70% of patients receiving budesonide, 280 micrograms/day, reported substantial or total control of symptoms, compared with 45% of placebo-treated patients. It is concluded that budesonide, 280 micrograms once daily, reduces polyp size and relieves symptoms in patients with nasal polyposis.