External jugular vein thrombosis is a rare complication that, when it occurs, is usually secondary to cervical trauma, infection, venous cannulation or malignancy. By contrast, spontaneous external jugular thrombosis is extremely uncommon. We report the case of a 69-year-old woman presenting to the Emergency Department with a 3 centimetre neck lump, which had suddenly appeared on the same day. She did not have any other relevant symptoms. The patient had not suffered any recent cervical trauma or infection. There was no personal or familial history of thromboembolic disease. Physical examination was normal, apart from the neck mass. A neck ultrasound revealed a non-occlusive thrombus inside the right external jugular vein. Usual workup, including a coagulation laboratory profile, autoimmunity and malignancy research, was unremarkable, identifying the thrombosis as idiopathic. Anticoagulation treatment was started, leading to a complete resolution of the thrombus, without recurrence. The sudden appearance of a painless cervical mass, without any identified triggering factor, is not common. The possibility of a jugular thrombosis should be kept in mind in the differential diagnosis of an isolated neck lump. In this case, point-of-care ultrasound, which is becoming increasingly available in Emergency Departments, contributed to the rapid and accurate diagnosis of the patient. LEARNING POINTS Vascular diseases such as aneurysms and thrombosis, although infrequent in jugular veins, should be considered in the differential diagnosis of a painless promptly growing neck lump. Since idiopathic external jugular vein thrombosis is uncommon, a complete diagnostic workout and follow-up is recommended, to exclude a triggering factor, such as an underlying malignancy, coagulation disorder or hidden infection. Ultrasound is the most accurate non-invasive method to diagnose venous thrombosis. Point-of-care ultrasound has proved to be a useful tool for the rapid diagnosis of vascular diseases.