In a double-blind, randomized, placebo-controlled clinical trial, the safety and efficacy of 900 mg/day sobrerol granules given for up to 10 days was assessed in 40 patients with chronic catarrhal rhinosinusitis. At the beginning of treatment a total of six patients had a fever, whereas body temperature was normal in all patients at the end of the treatment. Treatment with sobrerol significantly (P less than 0.01) reduced frontal headache and rhinorrhoea, efficacy being confirmed by rhinomanometry. Patients treated with placebo experienced an improvement in frontal headache, rhinorrhoea,and overall rhinomanometry scores. Treatment with sobrerol was well tolerated but two patients treated with placebo reported adverse reactions (stomach pain and cutaneous rash). These preliminary data suggest that sobrerol could be useful if administered with an anti-inflammatory drug for the treatment of chronic catarrhal rhinosinusitis.