OBJECTIVES: The benefit of adjuvant chemotherapy for early-stage breast cancer (BrCa) patients depends on baseline recurrence risk. Gene expression profiling (GEP) of tumours informs baseline risk prediction, potentially reducing unnecessary treatment and healthcare costs. Limited evidence exists on its clinical utility; we explored patients’ and oncologists’ perspectives on GEP in chemotherapy decisions. METHODS: We conducted individual interviews with medical oncologists (n=10), plus focus groups and individual interviews with BrCa patients (n=20) from Ontario, Canada. BrCa patients who underwent genomic testing (‘OncotypeDx’), were recruited through oncology clinics from two academic hospitals in the Greater Toronto Area. Medical oncologists were recruited through participating oncology clinics, professional advertisements and referrals from the research team. Data were analyzed using interpretative qualitative methods, including content analysis, qualitative description and constant comparison techniques. RESULTS: Patients and oncologists valued GEP as an additional decision-support tool, complementing existing clinical indicators, though perceived utility varied between patients and oncologists. Patients valued the test highly, suggesting it was one of the primary determinants of their treatment decision. All patients followed the course of action their results suggested. Patients with intermediate scores often used the results to reinforce their pre-existing treatment preferences. Oncologists were mixed about the test’s utility. Some considered it another tool supporting their approach to risk assessments; others used it more definitively to resolve their uncertainty. Oncologists explained the test’s contribution to decision-making but remained uncertain about patients’ understanding and expectations of the test. Some raised concerns about the variability of its use and interpretation within their medical community. CONCLUSIONS: Patients and oncologists valued the test, often using it as a primary determinant in their treatment decision, despite oncologists’ concerns about its technical limitations and patients’ limited understanding. Results identify need for informational decision aids and practice guidelines to support patient understanding and standardized application of the test.