OBJECTIVE: To assess possible ergogenic properties of corticosteroid administration. DESIGN: A balanced, double blind, placebo-controlled design was used. PARTICIPANTS: Twenty-eight well-trained cyclists and rowers. INTERVENTION: Four weeks daily inhalation of 800 microg budesonide or placebo. Main outcome measurements: The subjects performed three incremental cycle ergometer tests until exhaustion, before and after two and four weeks of placebo or budesonide administration, to measure maximal power output (Wmax). Once a week they filled in a profile of mood state (POMS) questionnaire. RESULTS: There was no significant difference in Wmax between the placebo (376+/-25 watt) and the corticosteroid group (375+/-36 watt) during the pre-intervention test, and there were no significant changes in either group after two and four weeks of intervention. No effect of the intervention on mood state was found. CONCLUSION: Four weeks of corticosteroid or placebo inhalation in healthy, well-trained athletes did not affect maximal power output or mood state. Hence no ergogenic properties of four weeks corticosteroid administration could be demonstrated, which corroborates previous studies with short-term corticosteroid administration.