Primary papillary carcinoma of a thyroglossal duct cyst: report of a case and literature review. Thyroglossal duct cysts are the most common anomaly in thyroid development. They are twice as frequent as branchial cleft abnormalities and, in children, are second only to enlarged cervical lymph nodes as the cause of neck mass. Generally, duct cysts are benign, but 1 per cent of cases may be malignant. From the world literature, 114 cases of malignant thyroglossal cysts were available for review. With the addition of our own case, we discuss 115 instances of duct cysts. The different types of neoplasia described included thyroid papillary carcinoma in 81.7 per cent, mixed papillary-follicular carcinoma in 6.9 per cent, squamous cell carcinoma in 5.2 per cent, follicular and adenocarcinoma in 1.7 per cent each, and malignant struma, epidermoid carcinoma and anaplastic carcinoma in 0.9 per cent each. Of the 115 cases surveyed, 35 thyroid glands were examined microscopically; of these, four (11.4 per cent) contained malignant foci. Whether these are primary malignancies of the thyroglossal duct cysts or metastases is discussed. 