The study was carried out to evaluate the therapeutic effects of zanamivir, a highly selective, potent and specific inhibitor of influenza A and B virus neuraminidases, in adult patients with acute influenza-like illness. Patients who presented within 36 h of the onset of influenza-like symptoms were randomly assigned to receive one of three treatments, twice daily, for 5 days: 10 mg zanamivir powder for inhalation (zanamivir inhalation group), 10 mg zanamivir powder for inhalation plus 6.4 mg zanamivir nasal spray (zanamivir inhalation plus intranasal group) or placebo (placebo group). The primary end point was the time to alleviation of the three major symptoms (fever, headache and myalgia). The secondary end point was the time to alleviation of five influenza symptoms (fever, headache, myalgia, cough and sore throat). One hundred and sixteen patients with influenza-like illness were recruited to the study. No differences were observed between the two groups of patients who received zanamivir (inhalation group or inhalation plus intranasal group). Patients who received zanamivir recovered significantly faster (median 3 days to recovery) than the patients in the placebo group (median 4 days to recovery; P < 0.01). Topically administered zanamivir was well tolerated. This study confirms that in adults, topically administered zanamivir is well tolerated and is effective in reducing the time to alleviation of influenza symptoms.