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As ultrasound is commonly used these days, more and more lesions are found incidentally, including simple cysts, isoechoic nodules, and hypoechoic nodules, such as ectopic thymus. We here report the prevalence of ectopic thymus and its natural course.We retrospectively reviewed the charts of 690 subjects (342 males, 348 females) who were under 18 years old and had undergone neck or thyroid ultrasound at Hallym University Sacred Heart Hospital from January 2014 to June 2017. Ectopic thymus was defined when the incidental lesion was hypo- to iso-echoic compared to adjacent muscles with vascularity and did not have a connection to the thymus in the mediastinum.Of the 690 children, 16 (9 males, 7 females; median and ranges of age at diagnosis, 13 months and 18 days to 10 years) were diagnosed as having ectopic thymus. The most common location of ectopic thymus was in the inferior aspect of the left thyroid lobe. The median and ranges of the longest diameter at the time of initial diagnosis was 22.5 mm and 8 mm to 38 mm, respectively. Four cases underwent follow-up ultrasound, of which 3 cases showed no change in size and 1 case showed decreased size.Ectopic thymus should be kept in mind when a lesion is found in the neck and thyroid incidentally, and physicians should reassure parents about its benign course.