Purpose:To compare the clinical findings,tear film function and impression cytology between patients with active and inactive Thyroid Eye Disease (TED).Methods:A total of 56 patients with TED and 30 controls were recruited in this prospective observational cohort study.TED patients were divided into active TED and inactive TED types according to a seven-point modified formulation of the Clinical Activity Score (CAS).All participants underwent full eye examinations including Ocular Surface Disease Index (OSDI) score,tear film break-up time (TBUT),fluorescein staining and Schirmer Ⅰ test.Thirty nine patients with thyroidassociated orbitopathy (TAO) received Nelson's grade with conjunctival impression cytology.Proptosis,palpebral fissure width and lagophthalmos were assessed.Results:Ocular surface parameters including proptosis,palpebral fissure width and iagophthalmos did not differ between active and inactive TED patients (P＞0.05). Both active and inactive TED patients obtained higher fluorescein staining scores, lower TBUT scores and significantly lower Schirmer test scores than those of controls (P＜0.001for all ).Additionally, the TBUT score was significantly lower and the OSDI score significantly higher in the active TED group compared with those in the inactive TED group (P＜0.001for both).Impression cytology revealed a higher proportion of grade 2-3 changes in the active TED group compared with the inactive TED group(P＜0.001).Conclusion:Orbital inflammation in TED patients may lead to decreased tear film stability and ocular surface squamous metaplasia.(Eye Science 2012 ; 27:64-68 )