Background: Supplier-induced demand (SID) refers to the concept that healthcare providers may deliver services to patients that are not medically necessary. An estimation of the extent to which this event has occurred can be insightful for policymaking and guiding health systems. This study aimed to investigate the extent to which SID presents itself during diagnostic MRI (magnetic resonance imaging) for primary breast cancer. Methods: In this cross-sectional study, data were obtained using questionnaires from a random sample of 310 cases. To identify patients who were candidates for undergoing a necessary diagnostic MRI, we employed the international clinical guidelines with the confirmation of our expert panelists. With their assistance, a comprehensive index was created to screen those who were affected by SID. Results: Of the respondents, 94.1% had undergone an unnecessary diagnostic MRI and, thus, were likely affected by SID, which indicated the lack of sovereignty of clinical guidelines in the prescription of MRI diagnosis imaging. Conclusion: This study supported the SID hypothesis and the unnecessary demand for diagnostic MRI in primary breast cancer. In addition, our evidence indicated that excessive costs were imposed; these can positively influence policymakers’ decisions regarding healthcare management.