Introduction. Pseudoaneurysms are an infrequent cause of palpable mass in the head and neck; usually secondary to invasive procedures; trauma and infectious causes are rare.  Clinical Case. We present the case of a 14-year-old boy who, after suffering a blunt contusive trauma in the left preauricular region surgically treated, develops a month later a pulsatile volume increase after a minor trauma in the area. Ultrasound shows the yin yang sign, indicator of pseudoaneurysm. The emptying of the pseudoaneurysm and subsequent vascular suture was performed without incident.  Discussion. Despite the low frequency of pseudoaneurysm as a cause of mass in the head and neck, it should be considered as a differential diagnosis in the context of post-traumatic pulsatile masses, from hours to days of evolution. The study of choice is Doppler ultrasound and the Gold Standard treatment is surgery with vascular suture.

Keywords: Pseudoaneurysm, yin yang sign, superficial temporal artery, vascular trauma, pulsatile mass.