1. Orbital accumulation of hydrophilic, interstitial glycosaminoglycans (GAGs) and subsequent expansion of retrobulbar tissue lead to the clinical manifestation of exophthalmos in patients with Graves' eye disease. 2. A highly specific method to determine the concentration and biochemical composition of different GAGs was developed in order to obtain a sensitive test system for the activity of the disease. By means of this method, GAG excretion in 24 h urine collections of 56 patients and 21 controls was analysed by precipitation with cetylpyridinium chloride and potassium acetate in ethanol, followed by sequential enzymic hydrolysis with chondroitin AC lyase, chondroitin ABC lyase and hyaluronate lyase, with HPLC analysis of the resulting α,β-unsaturated disaccharides by anion-exchange chromatography. 3. Concentrations of GAG, chondroitin sulphate A (CA), dermatan sulphate (DS) and hyaluronic acid (HA) were determined in patients and controls, with high recovery rates [72.2 ± 5.3%, mean ± SEM; detection limit, 4.2 μg/l (0.01 μmol/l)], revealing marked differences in urinary concentrations of total GAG and HA, as well as an elevation of CA in patients compared with controls. 4. Method sensitivity was 0.86 for patients with active Graves' eye disease, and 0.87 for patients with untreated ophthalmopathy, whereas specificity was 1.0 for patients with inactive disease. Patients with increased GAG concentration responded well to steroids and/or orbital irradiation (before therapy: GAG, 111.49 ± 40.32; CA, 59.58 ± 21.34; DS, 25.05 ± 8.12; HA, 26.88 ± 11.63 mg/24 h; during therapy: GAG, 54.22 ± 10.94; CA, 20.52 ± 4.58; DS, 17.65 ± 3.46; HA, 16.05 ± 3.69 mg/24 h), whereas GAG excretion increased markedly 2–3 months after stopping prednisone therapy in patients with still active eye disease (GAG, 109.9 ± 10.51; CA, 63.8 ± 7.34; DS, 24.1 ± 5.07; HA, 22.0 ± 6.28 mg/24 h). 5. This sensitive method determines the nature of renally excreted GAGs, reflecting the aberrant synthesis pattern of fibroblasts in patients with Graves' disease.