OBJECTIVES: Individuals with BrCa have high decisional conflict with respect to treatment decisions. GEP of tumours informs risk prediction, potentially affecting decisions about adjuvant chemotherapy in early BrCa, where only 15% will experience recurrence. We aimed to examine whether GEP reduces decisional conflict in chemotherapy treatment decision-making. METHODS: We embedded the validated Decisional Conflict Scale (DCS) into our discrete choice experiment survey examining preferences for chemotherapy treatment in early BrCa. Of the 1004 general population participants, 200 completed the DCS before (DCS-1; no GEP test score in scenario) and after (DCS-2; GEP test score added to scenario) the discrete choice experiment. The 16-item DCS was scored from 0-100 with five subscores. Mean total and subscores, standard deviations and change in scores were calculated, with significance based on matched pairs t-tests (p<0.05). We anticipated GEP would decrease decisional conflict in individuals unsure of their chemotherapy treatment decision. RESULTS: As anticipated, total score and all subscores (uncertainty, informed, values clarity, support, and effective decision) decreased significantly (all p<0.05) in the group of respondents (n=33) who indicated uncertainty about taking chemotherapy in DCS-1 but changed to no chemotherapy after receiving a GEP test score in DCS-2. In the group of respondents (n=25) who indicated they would undergo chemotherapy in DCS-1 but changed to unsure in DCS-2, their effective decision subscore increase significantly (24.5 to 34.5, p<0.05). In the overall sample (n=200), total decisional conflict decreased from DCS-1 to DCS-2 by 0.5 (p=0.3) and all subscores had non-significant decreases with the exception of effective decision, which had a non-significant increase. CONCLUSIONS: GEP influences chemotherapy treatment decisional conflict in individuals who are initially unsure in their treatment decision-making. However, we do not observe this effect in individuals who do not change their chemotherapy treatment decisions.