An increased accumulation of glycosaminoglycans (GAGs) in retrobulbar tissues has been reported in patients with Graves' ophthalmopathy (GO). We examined quantitative urinary GAGs excretion in patients with different grades of GO. 36 patients with Graves' disease were included in the study. They were classified according to their grade of ophthalmopathy: Grade 2 (n=14), Grade 3 (n=14) and Grade 4 (n=8). When GAG levels of patients with ophthalmopathy were compared to controls, a significant increase was observed in both Grade 3 and Grade 4 patients, whereas no significant difference was found in Grade 2 patients. These results show that urinary excretion of GAGs is a possible marker for the activity of Graves' ophthalmopathy. Determination of urinary level of GAGs is recommended as a simple laboratory method for estimating the grade of ophthalmopathy.