The aim of the study was to assess the efficacy and safety of nasal aqueous beclomethasone dipropionate (BDP), 400 micro g/day, given via a metered pump in a once-daily or twice-daily regimen following a double-blind, parallel group design over a 12-week period. Adult patients (n=112) with allergic or non-allergic chronic rhinosinusitis recorded their nasal and ocular symptoms for the 7-day run-in period and for the first 4 weeks of treatment. At baseline and after 4 weeks the airways' resistance via active anterior rhinomanometry and the volume and area section via acoustic rhinometry were measured. Morning serum cortisol was measured at baseline and at week 12. Adverse events were to be reported at each visit. Of the 112 randomised patients, three did not enter the ITT analysis and another 13 in total discontinued the treatment. Significant improvements over the baseline were reported in both groups for the primary variable sum of nasal scores (-53.7% in the once-daily group and -59.7 in the twice-daily group), as well as for each nasal and ocular symptoms, without differences between the groups. Because of a wider variability than expected, the 95% confidence interval (C.I.) for the difference between the least square means exceeded the pre-defined limit of +/-10% of the reference mean. Similar improvements in both groups were also reported for the nasal airway patency's parameters. The total number of drug-related adverse events was 26 in the once-daily group and 32 in the twice-daily group, with most of the events consisting of local effects at the site of application. No signs of adrenal suppression were observed, and serum morning cortisol values did not significantly change. The once-daily BDP dosing (400 micro g/day) therefore has a similar efficacy and safety profile as the same daily dose given in a twice-daily regimen.