OBJECTIVE: The Graves' ophthalmopathy quality of life questionnaire (GO-QOL) is the first instrument available to measure health-related quality of life (HRQL) of patients with Graves' ophthalmopathy. The main objective of this study was to define a minimal clinically important difference (MCID) in score on the GO-QOL that can be considered an important improvement in HRQL by examining changes in GO-QOL scores in patients who subjectively report improvement from their treatment. A secondary objective was to test the longitudinal validity of the GO-QOL, using prespecified hypotheses about expected treatment effects. DESIGN: A prospective cohort study. PATIENTS: We included 164 patients who were scheduled for radiotherapy (23), orbital decompression (10 for sight loss, 38 for exophthalmos), eye muscle surgery (31), eyelid lengthening (43) or blepharoplasty (19). MEASUREMENTS: Patients completed the GO-QOL and three general HRQL questionnaires, before and three or six months after treatment, depending on the performed procedure. Clinical characteristics were collected from the medical records. Mean changes in GO-QOL scores and effect sizes were calculated after different treatments, and in subgroups of responders and nonresponders according to clinical characteristics and according to the patients themselves. RESULTS: A clinical response to treatment was associated with a change in GO-QOL scores of approximately 10-20 points after major treatments (radiotherapy or decompression), and with a change of approximately 3-10 points after minor surgery (eye muscle surgery, eyelid lengthening, blepharoplasty). Changes in GO-QOL scores of about 6-10 points were considered important improvements by the patients themselves. The direction and amount of change in GO-QOL scores after different treatments were in accordance with our prespecified hypotheses about treatment effects. Effect sizes in the GO-QOL subscales were generally higher than effect sizes of the general HRQL subscales, supporting the longitudinal validity of the GO-QOL. CONCLUSIONS: As a general guideline, one could consider a mean change of at least 6 points on one or both subscales an important change in daily functioning for patients. For more invasive therapies, a change of at least 10 points is recommended as a minimal clinically important difference.