Preschool children with intermittent wheeze are often prescribed inhaled corticosteroids, although there is no proven benefit. Measurement of airway resistance by the interrupter technique can be used to objectively assess response to treatment. If lung function improves, treatment may be justified. Children with intermittent wheeze aged 2 to <or= 5 years of age completed a 6-week randomized controlled crossover trial of fluticasone propionate (100 micro g, twice daily), followed by a 10-week parallel extension. The relationships between changes in resistance, serum immunoglobulin E and sensitization measured by skin prick testing were investigated. Sixty-one children completed the crossover trial and 44 (72%) completed the extension. After 6 weeks, geometric mean change in resistance was -16.0% (95% confidence interval, -7.0 to -25.0%, p = 0.003) in sensitized children and -3.5% (95% confidence interval, +0.7 to -7.6%, p = 0.1) in nonsensitized children. Changes in resistance were unrelated to immunoglobulin E. Sixteen weeks after stopping fluticasone, resistance returned to baseline. This is the first study of preschool children with intermittent wheeze that has related changes in lung function on treatment to aeroallergen sensitization. Lung function improved in sensitized children and deteriorated after stopping treatment. Treatment with inhaled steroids may be justified in sensitized children.