Medullary thyroid carcinoma (MTC) is a calcitonin (CT)-secreting endocrine tumor. Although plasma CT level is a specific and sensitive marker of MTC, its preoperative usefulness in predicting tumor size and postoperative CT normalization has not been documented. From a nationwide database set up by the French CT Tumor Study Group, 226 MTC patients were selected according to the following criteria: preoperative CT level determination by an immunoradiometric assay (normal value, < 10 pg/mL) within the 6 months prior to surgery, total thyroidectomy and diagnosis of MTC ascertained by histological report including tumor size. Patients were 129 females and 97 males (female/male ratio, 1.3). One hundred and twelve patients (49.6%) had the sporadic variety of the disease, 74 (32.7%) had multiple endocrine neoplasia 2A, three (1.3%) had multiple endocrine neoplasia 2B, and 37 (16.4%) had familial MTC. Median age at diagnosis was 44.8 yr (range, 4.9-80.1 yr). Complete neck dissection was performed in 159 patients (70.4%). Postoperative CT normalization was ascertained by negative response of CT to pentagastrin stimulation (< 10 pg/mL) in 94 patients. Seventy-one patients were considered as not cured because of residual tumor tissue and/or elevated CT levels. Median tumor size was 11.0 mm (range, 0.2-80.0 mm), significantly larger in females (15.0 vs. 8.0 mm, P < 0.05), and in sporadic forms (15.0 vs. 7.0 mm, P < 0.05). Tumor size was significantly correlated (r2 = 0.52, P < 0.01) with preoperative CT levels, the relationship being more straight in familial (r2 = 0.71) than in sporadic (r2 = 0.36) forms. Furthermore, preoperative CT levels under 50 pg/mL appeared to be predictive of postoperative CT normalization (44 of 45 patients). However, higher CT levels did not mean absence of postoperative CT normalization (50 of 120 patients). We conclude that low preoperative CT levels are predictive of tumor size and postoperative CT normalization.