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The placebo effect-a change attributable only to an individual's belief in the efficacy of a treatment-might provide a worthwhile improvement in physical performance. Although sports scientists account for placebo effects by blinding subjects to treatments, little research has sought to quantify and explain the effect itself. The present study explored the placebo effect in laboratory cycling performance using quantitative and qualitative methods.Six well-trained male cyclists undertook two baseline and three experimental 10-km time trials. Subjects were informed that in the experimental trials they would each receive a placebo, 4.5 mg·kg−1 caffeine, and 9.0 mg·kg−1 caffeine, randomly assigned. However, placebos were administered in all experimental conditions. Semistructured interviews were also conducted to explore subjects' experience of the effects of the capsules before and after revealing the deception.A likely trivial increase in mean power of 1.0% over baseline was associated with experimental trials (95% confidence limits, −1.4 to 3.6%), rising to a likely beneficial 2.2% increase in power associated with experimental trials in which subjects believed they had ingested caffeine (−0.8 to 5.4%). A dose-response relationship was evident in experimental trials, with subjects producing 1.4% less power than at baseline when they believed they had ingested a placebo (−4.6 to 1.9%), 1.3% more power than at baseline when they believed they had ingested 4.5 mg·kg−1 caffeine (−1.4 to 4.1%), and 3.1% more power than at baseline when they believed they had ingested 9.0 mg·kg−1 caffeine (−0.4 to 6.7%). All subjects reported caffeine-related symptoms.Quantitative and qualitative data suggest that placebo effects are associated with the administration of caffeine and that these effects may directly or indirectly enhance performance in well-trained cyclists.