The simultaneous occurrence of two neoplasms of different cellular origin in one organ is a known but rare event. Such a situation occurs in the thyroid when medullary and follicular carcinoma with differentiation develops. Several cases of transitional tumors have been reported, but the simultaneous occurrence of a carcinoma with medullary and follicular differentiated carcinoma is rare. In all patients, tumors were suspected after local examination of the thyroid. We report on a patient in whom a papillary microcarcinoma was detected on the left side and a medullary carcinoma on the right side while the patient was undergoing surgery for secondary hyperparathyroidism. Both tumors were confirmed by immunohistology; regional lymph nodes were free of tumor. Although the simultaneous occurrence of papillary and medullary carcinoma may have been a simple coincidence, the patient's history offers room for further speculation; he had chronic replicative hepatitis C and a 13-yr history of immunosuppression following renal transplantation. Therefore, a common oncogenic stimulus may have been involved. In the final analysis, the reason for both malignancies could not be clearly established.