Two hundred and forty-one children with perennial asthma had their normal dose of budesonide (administered via Nebuhaler) reduced by 50%. Asthma control deteriorated in 126 patients to such an extent that budesonide had to be increased to the normal dose. During a subsequent two week run-in these 126 patients were treated with their normal dose of budesonide via Nebuhaler. Thereafter 64 were randomized to treatment with their normal budesonide therapy and 62 to treatment with half their normal budesonide dose via Turbuhaler for nine weeks. Asthma symptoms, peak-flow measurements, and beta 2-agonist use were recorded. Pulmonary function tests, exercise tests and 24 hour urine sample collections were performed. There were no differences between the groups in any of the parameters studied during run-in or during the study period, except for use of beta 2-agonist, which was significantly lower in the Turbuhaler-treated group. We conclude that Turbuhaler is more effective than Nebuhaler in the treatment of asthma. Therefore the dose of budesonide should be reduced when patients are switched from Nebuhaler to Turbuhaler treatment.