Tissue levels of thyroglobulin (Tg) or calcitonin were compared with specimens from neck lymph node biopsy in patients with suspected recurrent differentiated (papillary or follicular) or medullary thyroid cancer. Thirty-six neck lymph node biopsies were performed in 29 patients. Tissue Tg levels were obtained from 31 specimens from patients with differentiated thyroid cancers, and tissue calcitonin levels were obtained from five specimens from patients with medullary cancer. Thirteen nodes were diagnosed as negative for cancer at surgery (n = 3) or follow-up sonography (n = 10). Malignant disease was confirmed at surgery in 23 of the 36 lymph nodes. Cytopathologic examination had a sensitivity of 91% and a specificity of 100%. Tissue Tg levels ranged from 0 to 3.5 ng/mL (mean, 1.5 ng/mL; median, 1.2 ng/mL) in 12 of the 13 benign lymph nodes and from 21 to 247,500 ng/mL (mean, 30,600 ng/mL; median, 2,330 ng/mL) in the 23 malignant nodes. Tissue calcitonin levels were elevated (range, 850-703,125 pg/mL; mean, 184,762 pg/mL; median, 17,538 pg/mL) in four malignant nodes and were normal (3.0 pg/mL) in one benign node. Diagnostic sensitivity of tissue markers was 91%. Specificity was 91%. The combined diagnostic sensitivity and specificity of tissue marker analysis and cytopathologic examination was 100%.