Thirty-eight patients (25 women, 13 men; mean age, 57.8 [32 to 91]) showing one or more medullary thyroid microcarcinomas (ie, < 1 cm), with no prior MEN II or medullary thyroid carcinoma history in their family, were reviewed. Follow-up was available for 29 patients (mean, 53.6 months [1 to 147]). 21 patients (72.4%) are alive and free of disease, four patients (13.8%) died during follow-up without disease, 2 patients are alive with disease (local recurrence and persistent hypercalcitoninemia) after 80 and 99 months, respectively, and 2 patients died of disease after 24 and 46 months. Most tumors were incidental pathological findings (19 of 38) or were discovered by systematic blood calcitonin measurement for a nodular thyroid disease (15 of 38). Only the four patients who had an unfavorable outcome were symptomatic cases (palpable micro-MTC, diarrhea, cervical lymph node metastasis and pulmonary metastatic disease). The two patients with metastatic disease at diagnosis died during follow-up. In univariate analysis, a symptomatic medullary thyroid carcinoma was a strong predictor of an unfavourable outcome (p < .00008), as were the preoperative calcitonin level (P = .007) and an elevated postoperative calcitonin level (P = .004). Among 30 histopathological criteria, only the presence of amyloid correlated with an unfavorable outcome (P = .018).