Local recurrence in differentiated and medullary thyroid carcinoma develops frequently from metastatic infiltration of cervical lymph nodes. Despite an aggressive surgical approach, postoperative calcitonin levels as biochemical evidence for residual cancer cells remain often elevated in patients with medullary thyroid carcinoma. In the present study, we compared the detection rates of disseminated medullary thyroid carcinoma cells in cervical lymph nodes by histopathology with reverse transcription-polymerase chain reaction (RT-PCR) amplification of cytokeratin 20 (CK20) transcripts as a more sensitive but still specific molecular parameter for residual thyroid cancer cells. Forty-two cervical lymph nodes obtained from 7 patients with CK20positive medullary thyroid carcinomas were cut into two halves, one used for conventional histology, the other subjected to RNA extraction and subsequent amplification of cytokeratin 20 transcripts. Matching results for CK20 RT-PCR and histopathology were found in 74% (31/42)of the examined lymph nodes (52% positive results, 48% negative results). Positive CK20 RT-PCR pointed to residual thyroid carcinoma cells in another 19% (8/42), in which no thyroid carcinoma cells were identified by histopathology. Histology and immunohistochemistry,however, identified tumor cells in 7% (3/42) of the analyzed lymph nodes, from which no CK20 transcript could be amplified (false-negative results). These data suggest that CK20 RT-PCR might be more sensitive to detect nodal involvement of CK20 positive medullary thyroid carcinomas than conventional histopathology. In combination with histology, it might help to identify patients with residual disease after surgery.