A 30-year-old man was admitted to our hospital because of fatigue, palpitation and severe weakness of both legs. The admission laboratory findings revealed thyrotoxicosis, and 131I thyroid scintigraphic imaging revealed a low radioactive iodine uptake. He was treated for painless thyroiditis for about 4 months. However, thyrotoxic state had continued and radioactive iodine uptake was markedly increased in the follow up scan. Painless thyroiditis often relapses, but rarely develops into Graves' disease. This is a rare case in which painless thyroiditis was followed by Graves' disease.