Polycystic ovary syndrome (PCOS) and Graves' disease are the common causes of menstrual irregularity leading to infertility in women of child-bearing age. A 21-year-old female patient visited us with complaints of oligomenorrhea and hand tremor. She was diagnosed as having PCOS and hyperthyroid Graves' disease, simultaneously. She had low body weight (BMI: 16.4 kg/m(2)), mild hirsutism, and thyrotoxicosis. The patient was treated with anti-thyroid drug and beta-blocker for about two years, and then recovered to normal thyroid function. Although some studies have suggested a connection between PCOS and autoimmune thyroiditis, no study indicated that PCOS is associated with Graves' disease until now. Here, we describe the first case report of a lean woman with normal insulin sensitivity presenting PCOS and Graves' disease simultaneously.