BACKGROUND Graves disease is occasionally associated with thyroid cancer. The most common histological type of thyroid cancer in patients with Graves disease is papillary followed by follicular. Medullary thyroid cancer and Graves disease have been reported simultaneously only in a few cases in the literature. CASE REPORT A case of coexistence of Graves disease and medullary thyroid cancer is described in this report. The patient was diagnosed with Graves disease 8 years ago. Although he had an initial successful treatment with carbimazole, in the last 2 years no steady euthyroid function was achieved. Total thyroidectomy was considered as the optimal treatment. An incidental medullary microcarcinoma with maximum diameter 0.5 cm was identified by pathology report. CONCLUSIONS Medullary thyroid cancer has been reported in patients with Graves disease in 15 cases, including the current case. Medullary thyroid cancer is aggressive, and a delayed diagnosis would be harmful. Hence, patients with Graves disease should be evaluated regularly by a thyroid specialist.