Prediction of remission in medullary thyroid carcinoma (MTC) depends on histopathological information often unavailable before surgery. Simply requiring a venous blood sample, preoperative basal calcitonin levels may be a better indicator of remission. In this institutional series of 224 consecutive patients with MTC and elevated preoperative basal calcitonin levels, postoperative calcitonin levels normalized in 28 (62%) of 45 patients with node-negative MTC and in 18 (10%) of 177 patients with node-positive MTC. On multivariate analysis, preoperative basal calcitonin levels greater than 500 pg/ml best predicted the failure to achieve biochemical remission, followed by nodal metastasis and reoperative status. Cumulative rates of biochemical remission fell continuously with rising serum basal calcitonin in node-negative patients. Node-positive patients did not achieve biochemical remission when their preoperative basal calcitonin levels exceeded 3000 pg/ml. Nodal metastasis started emerging at basal calcitonin levels of 10-40 pg/ml (normal range, <10 pg/ml). Distant metastasis and extrathyroidal growth began appearing in patients with node-positive MTC at basal calcitonin levels of 150-400 pg/ml. There were no differences between patients with sporadic and hereditary MTC after adjusting for multiple testing. Preoperative basal calcitonin levels may thus help individualize the extent of surgery and postoperative follow-up intervals for MTC.