Introduction: Determining the likely benefit of adjuvant chemotherapy for early-stage breast cancer patients depends on estimating baseline recurrence risk. Gene expression profile (gep) testing of tumours informs risk prediction, but evidence of its clinical utility is limited. We explored patient perceptions of gep testing and the impact of those perceptions on chemotherapy decisions. Methods: We conducted one focus group (n = 4) and individual interviews (n = 24) with patients who used gep testing, recruited through clinics at two hospitals in Ontario. Data were analyzed using content analysis and constant comparison techniques. Results: Patients’ understanding of gep testing was variable, and misapprehensions were common. Patients valued the test because it provided them with certainty amidst confusion, with options and a sense of empowerment, and with personalized, authoritative information. They commonly believed that the test was better and fundamentally different from other clinical tests, attributing to it unique power and truth-value. This kind of “magical thinking” was derived from an amplified perception of the test’s validity and patients’ need for reassurance about their treatment choices. Despite misperceptions or magical thinking, gep was widely considered to be the deciding factor in treatment decisions. Conclusions: Patients tend to overestimate the truth-value of gep testing based on misperceptions of its validity. Our results identify a need to better support patient understanding of the test and its limitations. Findings illustrate the deep emotional investment patients make in gep test results and the impact of that investment on their treatment decisions.