The study compared visual evoked potentials of patients with uncomplicated Graves' ophthalmopathy, patients with ophthalmopathy and elevated intraocular pressure or suspect glaucoma, and patients with dysthyroid optic neuropathy (DON). The aim of the study was to investigate the clinical potential for the visual evoked potentials (VEP) in the differential diagnosis among the groups. The VEPs were obtained from 43 subjects with endocrine ophthalmopathy. Group I included patients with uncomplicated ophthalmopathy (30 eyes); group II included patients with ophthalmopathy, intraocular pressure > or = 23 mmHg with and without early visual field defects, and no evidence of apical crowding on coronal computed tomography scan (28 eyes); group III included patients with DON (28 eyes). Amplitude and latency of major component of pattern VEP were obtained at three visual angles (60', 30', 15'). Data from each group was compared with data from age-matched normal subjects. Disturbances of VEP were found mainly in patients of Group II and Group III. Control Group had normal VEP. About the differential diagnosis between Group II and Group III the most important parameter was the N75-P100 amplitude for 15' of pattern stimulation. Only for this visual angle, Group II and Group III had not overlapped N75-P100 amplitude. This study shows that VEP detect visual function abnormalities noninvasively in patients with complicated Graves' ophthalmopathy. Results also indicate the clinical potential for VEP in the differential diagnosis between patients suffering from ophthalmopathy complicated by ocular hypertension or suspect glaucoma and patients with dysthyroid optic neuropathy.