Patients with neurofibromatosis type 1 not only have characteristic skin findings but are also known to have vascular disorders due to vascular vulnerability. A 44-year-old man with previously undiagnosed neurofibromatosis type 1 was brought to the emergency room due to a sudden subcutaneous hematoma with no history of trauma. Angiography revealed extravasation from the parietal branch of the right superficial temporal artery, which was embolized with n-butyl-2-cyanoacrylate. However, the next day, the patient exhibited an increased subcutaneous hematoma, and new extravascular leakage was detected at the frontal branch of the superficial temporal artery, which was also embolized with n-butyl-2-cyanoacrylate. The patient had physical findings characteristic of neurofibromatosis type 1, such as café-au-lait spots, and was subsequently diagnosed with neurofibromatosis type 1. No obvious neurofibroma or any other subcutaneous lesion associated with neurofibromatosis type 1 was identified in the affected area. Massive idiopathic arterial bleeding in the scalp, although infrequent, can be fatal. Neurofibromatosis type 1 should be considered when a subcutaneous scalp hematoma is observed without a history of trauma, even if the facial skin structure appears normal. Neurofibromatosis type 1 is also known to have multiple sources of hemorrhage. Thus, it is important to repeatedly evaluate vascular structures via cerebral angiography, contrast-enhanced computed tomography, and magnetic resonance imaging, if necessary.