Abstract: Objectives: To undertake a quantitative evaluation of a theory-based, interactive online decision aid (BresDex) to support women choosing surgery for early breast cancer (Stage I and II), based on observations of its use in practice.Methods: Observational cohort study. Website log-files collected data on the use of BresDex. Online questionnaires assessed knowledge about breast cancer and treatment options, degree to which women were deliberating about their options, and surgery intentions, pre- and post-BresDex.Results: Readiness to make a decision significantly increased after using BresDex (p<.001), although there was no significant improvement in knowledge. Participants that were ‘less ready’ to make a decision before using BresDex, spent a longer time using BresDex (p<.05). Significant associations between surgery intentions and choices were observed (p<.001), with the majority of participants going on to have BCS. Greater length of time spent on BresDex was associated with stronger intentions to have BCS (p<.05).Conclusion: The use of BresDex appears to facilitate readiness to make a decision for surgery, helping to strengthen surgery intentions.Practice implications: BresDex may prove a useful adjunct to the support provided by the clinical team for women facing surgery for early breast cancer.