Midline cervical cysts arising from the thyroglossal duct are one of the most common causes of anterior neck swelling in children. The potential for confusion between the thyroglossal duct cyst and the ectopic thyroid gland is well documented and may result in serious complications. A retrospective chart analysis was therefore undertaken to determine the relative importance of clinical evaluation, thyroid function testing, and radioisotope scanning in distinguishing these two entities preoperatively. A combined study was carried out by The Children's Hospital, Boston, Mass, and Rainbow Babies Hospital, Cleveland, Ohio. The hospital records, including nuclear scans, were reviewed. A total of 229 patients were taken to the operating room between January 1, 1978, and December 31, 1987. Nuclear scanning was performed in 30% of these patients. This subgroup of patients forms the basis for our guidelines of preoperative assessment. The absolute need for a preoperative scan is reexamined.