The aim of this study was to investigate whether budesonide, for 10 d, administered at the first sign of an upper respiratory tract infection, could reduce asthma symptoms in 1-3-y-old children with asthma during infections. The primary efficacy variable was symptom scores. The study had a multicentre, randomized, double-blind, placebo-controlled design with parallel groups. Fifty-five children with a mean age of 26 months received either budesonide or placebo via a spacer with a facemask. Each child was monitored for 1 y. Budesonide was given 400 microg q.i.d. for the first 3 d and b.i.d. for 7 d. Symptoms (cough, wheeze, noisy breathing and breathlessness) were scored (0-3) daily by the parents. Asthma symptom scores were lower in children treated with budesonide than in those given placebo. The effect was most pronounced for cough and noisy breathing, but it did not affect the need for hospital care. In conclusion, treatment with budesonide, started at the first sign of a respiratory infection, reduced asthma symptoms in toddlers with episodic asthma.