Objectives To quantify and discuss the prevalence of unsuspected thyroid lymph node metastases discovered in specimens from neck dissection for head and neck squamous cell carcinoma (HNSCC) and discuss the impact on patient management. Study design Retrospective study between May 2004 and January 2007. Setting University hospital. Methods Pathological analysis of cervical lymph node dissection performed during surgery for HNSCC in a total of 349 neck dissections in 266 consecutive patients. Results Twenty-one patients showed metastatic lymph nodes from thyroid cancer (prevalence 7.9%): 13 cases were metastatic from a papillary thyroid carcinoma and 8 cases from a follicular carcinoma. In 5 of the 21 patients, classical dissection was associated to recurrent nerve dissection and unilateral lobectomy; no thyroid carcinoma was found. Thirteen patients received radiotherapy for HNSCC. Follow-up comprised annual ultrasonographic examination of the neck and thyroid in these 21 patients. Total thyroidectomy was decided on in 5, with discovery of 3 micro-papillary thyroid carcinomas, in a single patient (complementary 131I treatment). No thyroid carcinomas were found for the other 4 patients. No patients died from thyroid carcinoma during follow-up (mean: 41 months). Conclusion The prevalence of lymph node metastasis from thyroid carcinoma in cervical lymph node dissection during treatment of HNSCC seems higher (7.9%) than rates reported in the literature (0.3 to 1.6%). This may be due to the histopathological methods employed. Management of patients should be discussed in the light of thyroid ultrasonography and prognosis of HNSCC.