A 45 year-old male medullary thyroid carcinoma(MTC)patient is presented to demonstrate the importance and usefulness of serial thallium-201(201TI)scans before and after operation. Four serial 201TI scans clearly showed the location and extension of the tumor before the operation, the residual tumor soon after the operadion, and the progression of the tumor even under sandostatin therapy and radiotherapy. The roles of 201TIscan in MTC and a residual tumor which was found soon after operation are of interests.