This study was performed to determine clinical features of dysthyroid optic neuropathy (DON) across Europe.Forty seven patients with DON presented to seven European centres during one year. Local protocols for thyroid status, ophthalmic examination and further investigation were used. Each eye was classified as having definite, equivocal, or no DON.Graves' hyperthyroidism occurred in the majority; 20% had received radioiodine. Of 94 eyes, 55 had definite and 17 equivocal DON. Median Clinical Activity Score was 4/7 but 25% scored 3 or less, indicating severe inflammation was not essential. Best corrected visual acuity was 6/9 (Snellen) or worse in 75% of DON eyes. Colour vision was reduced in 33 eyes, of which all but one had DON. Half of the DON eyes had normal optic disc appearance. In DON eyes proptosis was > 21 mm (significant) in 66% and visual fields abnormal in 71%. Orbital imaging showed apical muscle crowding in 88% of DON patients. Optic nerve stretch and fat prolapse were infrequently reported.Patients with DON may not have severe proptosis and orbital inflammation. Optic disc swelling, impaired colour vision and radiological evidence of apical optic nerve compression are the most useful clinical features in this series.