General health-related quality of life is markedly impaired in patients with Graves' ophthalmopathy (GO), and even worse than in patients with other chronic conditions like diabetes, emphysema or heart failure. A disease-specific quality-of-life questionnaire for GO has been developed, the so-called GO-QOL, consisting of two subscales: one for visual functioning (8 questions referring to limitations due to decreased visual acuity and/or diplopia) and one for appearance (8 questions referring to limitations in psychosocial functioning due to changes in appearance). The GO-QOL was found to be a valid and reliable instrument. A minimal clinically important difference (MCID) in the GO-QOL score was derived from data obtained before and after specific eye treatments. Based on the patient's opinions, changes of > or = 6 points (minor surgery) or > or = 10 points (surgical decompression, immunosuppression) are recommended as MCID. It is concluded that the GO-QOL is an useful instrument for measuring changes over time in visual functioning and appearance of GO patients. The GO-QOL is available in six languages, and can be used as a separate outcome measure in clinical studies.