Long-term course of patients with persistent hypercalcitoninemia after apparent curative primary surgery for medullary thyroid carcinoma. Thirty-one patients with persistent hypercalcitoninemia after seemingly adequate primary operation for medullary thyroid carcinoma (MTC) were followed for a mean period of 11.9 years after operation. Ten patients had sporadic MTC and the remaining patients were members of families with multiple endocrine neoplasia (MEN)--either MEN 2A (15 patients) or MEN 2B (six patients). Overall 5- and 10-year survival rates were 90% and 86%, respectively. Only four patients died at the completion of the study: two of MTC and two of unrelated causes. Eleven patients (35.5%) underwent surgical re-exploration after demonstration of recurrent disease clinically or radiologically. In no patient did the calcitonin level return to normal after re-exploration. The presence of more than three metastatic nodes at the time of initial operation was a statistically significant (p = 0.003) predictor for disease recurrence. Factors approaching statistical significance were patients younger than age 35 (p = 0.06) and the percentage of cells in the S phase of cell division (0.07). This data supports a conservative surgical philosophy in the management of the patient with persistent hypercalcitoninemia after resection of MTC. 