Questions: Can an education booklet handed out by a physiotherapist improve bra knowledge and fit, and level of breast support, of bras worn by adolescent female athletes? Design: Cluster-randomised trial with intention-to-treat analysis. Participants: 115 adolescent females from four regional sporting academies aged 16 yr (SD 1) and with an average Australian bra size of 12B. Intervention: The experimental group received an education booklet on bra fit and breast support from a sports physiotherapist. The control group received no intervention. Outcome measures: The primary outcome was bra knowledge measured by a questionnaire. Secondary outcomes were a pass on the Bra Fit Assessment and the Level of Breast Support tests, and breast discomfort during exercise rated on a 10-cm visual analogue scale. Results: Four months after receiving the education booklet, the experimental group had improved their bra knowledge 19% (95% CI 14 to 25) more than the control group. In addition, 39% (95% CI 19 to 54) more of the experimental group passed the Bra Fit Assessment test and 30% (95% CI 11 to 47) more passed the Level of Breast Support test than the control group at 4 months. There was no difference in the level of breast discomfort during exercise. Conclusion: Bra knowledge, bra fit, and level of breast support in adolescent female athletes were all poor but improved significantly after receiving an education booklet about breast support designed specifically for them.