This study aimed to detect the effect of influenza vaccination on quality of life, symptomatology and spirometry in asthmatic children. A randomised double-blind placebo-controlled trial in 696 (296 in 1999-2000 and 400 in 2000-2001) asthmatic children aged 6-18 yrs, which were vaccinated with either vaccine or placebo, was performed. Children participated for only one influenza season. They recorded symptoms in a diary and reported when symptom scores reached a predefined severity level. If this occurred research nurses visited them twice, first to take a pharyngeal swab and spirometry, and a week later to assess quality of life over the past illness week. Compared with placebo, vaccination improved health-related quality of life in the weeks of illness related to influenza-positive swabs. However, no effect was found for respiratory symptoms recorded in the diaries during those weeks. Similarly, no differences were found for quality of life in all weeks of illness or for respiratory symptoms throughout the seasons. Influenza vaccination was found to have a moderately beneficial effect on quality of life in influenza-positive weeks of illness in children with asthma.