It has become evident in recent years that parathyroid adenoma and well differentiated thyroid cancer occur together more than would be expected by chance alone. However, the association between them is not well understood. We have experienced 4 cases of coexistent parathyroid adenoma and well-differentiated thyroid cancer during the past 16 years. None of them had a familial incidence or a history of radiation exposure. Three cases showed symptomatic hypercalcemia(including renal stones, bone pain, joint pain) and in two of them(patient 1 and patient 2), thyroid abnormalities were detected preoperatively by neck ultrasonography or neck CT for evaluation of parathyroid lesions. However, in patient 3, a parathyoid humor was identified and removed incidentally during the course of thyroidectomy. In 3 cases, surgeries for thyroid carcinoma and parathyroid adenoma were performed during the same exploration of the neck, but in patient 4, thyroidectomy preceded parathyroidectomy; The interval between thyroidectomy and subsequent parathyroidectomy was 11 yeras. The thyroid tumors in 3 cases were papillary carcinoma, the sizes of which ranged from 1.0 cm to 1.5 cm in greatest diameter. The remaining case(patient 4) was minimal invasive follicular carcinoma. Total or near-total thyroidectomy with various types of cervical lymphnode dissection and bilateral neck exploration for the parathyroid lesion was performed in 3 cases with papillary carcinoma. Ipsilateral lobectomy and contralateral partial thyroidectomy with consequent unilateral neck exploration for the parathyroid tumor was performed in the case of follicular cancer. In our experience, parathyroid adenoma and well-differentiated thyroid carcinomas can be coexistent and we felt that the attention to the hypercalcemic patients would be needed for detection of this rare condition.