Mobile atheromatous plaque at the origin of the internal carotid artery (ICA) is a rare finding at the carotid ultrasound examination. Acute thrombosis is unusually seen. Either may be secondary to plaque hemmorhage, plaque ulceration, carotid dissection, or proximal embolus. We report an 89-year-old male suffering from an acute neurologic event who on ultrasound had a flapping heterogeneous atheromatous plaque at the right ICA origin. This mobile atheromatous plaque is distinguished from acute thrombus by its echogenicity, narrow pedicle of attachment, and absence of underlying intima. Subsequent computed tomography angiography and repeat ultrasound within 24 hr demonstrated the disappearance of this plaque. Only three small intimal flaps remained. In the intervening period, the patient s symptoms significantly worsened. While conservative management of thrombus with intravenous thrombolysis has been shown in some settings to be appropriate, the diagnosis and management of mobile atheroma are unclear. This case suggests that differentiation between atheroma and thrombus may be valuable in determining management strategies.