In 67 patients with medullary thyroid carcinoma, serum calcitonin levels were measured before (S1) and after (S2) surgical therapy. Doubling time of serum calcitonin levels (T2) was calculated in each patient with elevated calcitonin levels after surgery. Residual tumor weight (W2) was estimated as W1S2/S1-S2) where W1 was resected tumor weight. Reduction index (alpha) is defined as S2/S1 = (1/2) alpha. alpha T2 indicates expected prolongation in survival (EPS) by surgical therapy. Expected duration of survival after surgery (EDS) is estimated as beta T2, where beta times of doubling of the residual tumor (W2) make 1000 g of tumor which would generally kill the host. Death within 3 years after surgery or recurrence within 5 years was best associated with short EPS followed by short T2 or small alpha. Postoperative calcitonin levels had a rather weak correlation with the prognosis, and preoperative calcitonin levels had almost no correlation with the prognosis during the present observation period. All 3 patients with EDS shorter than 3.9 years died within 2 years and 2 months after surgery. These parameters allow quantitative judgement of the surgical benefit and quantitative prediction of the prognosis in each individual patient.