INTRODUCTION: Pseudoaneurysm of the breast is a very rare complication, which may occur after breast biopsy with a large core needle. We report a case of pseudoaneurysm of the breast that occurred after biopsy of a breast mass. CASE REPORT: The patient underwent core biopsy of a breast mass, progressing with extensive hematoma after the initial firing of the biopsy gun. Regression of the hematoma occurred but a nodular area persisted. A pseudoaneurysm was detected on ultrasonography and magnetic resonance imaging. DISCUSSION: An extensive breast hematoma following core needle biopsy usually regresses completely within a few weeks. The persistence of a vascularized lesion on ultrasound should draw attention to the diagnosis of pseudoaneurysm. This condition may be managed by endovascular procedure or surgical resection of the lesion.