RESUMEN
Ectopic cervical thymus is a rare cause of neck masses and can be found anywhere along its embryologic tract of descent from the angle of the mandible to the superior mediastinum, and it should be included in the differential diagnosis of neck masses, especially in children. Because most cases are not symptomatic, usually remains without diagnosis. Such anomalies are rarely diagnosed pre-operatively and often revealed by pathologic examination after operation. We introduced a 12-years-old girl with a soft left cervical mass anterior to Sternocleidomastoid muscle, from four months prior to refer, with firm consistency in touch and mild pain. With the initial diagnosis of lymphadenopathy was treated with antibiotics but did not respond to treatment. The patient underwent complete excision of the lesion and histological examination that showed an ectopic thymic cyst. This anomaly rarely diagnosed before surgery and can be mistaken easily with other cervical masses. Surgery is curative and the disease has excellent prognosis