Urinary excretion of glycosaminoglycans was measured in 10 patients with pretibial myxoedema, 7 of whom also had thyroid-associated ophthalmopathy, and in 3 additional patients with ophthalmopathy but no skin changes. Total uronic acid excretion was raised above control levels in only 2 patients, who had both eye and skin disease of recent onset. In these patients excretion was initially three times the control level but declined sharply in subsequent months. This decline was in the absence of effective treatment or spontaneous improvement and would appear to reflect the natural history of the disease. These data show that although glycosaminoglycans excretion may be disturbed in Graves' disease, it provides an unreliable reflection of clinical status and of the effectiveness of treatment.