We determined the frequency of clinical thyroid ophthalmopathy in Iranian patients. This cross-sectional study was performed at the Endocrinology Institute of Tehran University of Medical Sciences. All patients with documented thyroid disorders from September 2003 to July 2005 were recruited. Eye examinations included evaluation of soft tissue changes, measurement of proptosis, lid width, lagophthalmos, evaluation of eye muscle function, and determination of visual acuity. The activity of ophthalmopathy was scored according to the NOSPECS scale. Among 851 visited patients, 303 cases had thyroid eye disease (TED). The nature of the ophthalmopathy breaks down as follows: 53.4% were hypothyroid (9.3% Hashimoto disease), 5.5% euthyroid and 41.1% had Graves' disease. The prevalence of TED in males was 60% which was nearly 2 times the prevalence of TED in females. There was a significant relationship between presentation of TED and active smoking of the patients (P<0.0001) while no significant relationship with passive smoking was obtained (P=0.181). The most common clinical sign of TED in descending order respectively were proptosis with 63.4%, soft tissue involvement (40.9%), extraocular muscle involvement (22.1%), corneal involvement (12.9%) and optic nerve dysfunction (6.3%). Myasthenia gravis occurred in only 2 patients. In the logistic regression, occurrence of TED was influenced by cardiovascular disease (OR=5.346), Graves' disease (OR=47.507), radioiodine therapy (OR=2.590), and anti-thyroid medications (OR=0.650). Thyroid ophtalmopathy (orbitopathy) is a matter of important health concern among patients with thyroid disorder. Since TED occurred with a high prevalence in all thyroid states, a close collaboration between endocrinologists and ophthalmologists along with timely referrals of patients with any eye complaint is deemed necessary. Also smoking was the most important risk factor for developing TED. Therefore, it is advisable to raise awareness and to strongly encourage smokers with thyroid disorders to quit smoking.