The function of the corneal epithelium was evaluated in 30 patients with Graves' ophthalmopathy in whom no signs of corneal disease were observed. This function was evaluated by determining the permeability values of the corneal epithelium for fluorescein using fluorophotometry. The permeability values of the patients were compared with those of 46 healthy individuals. A significant increase in the permeability values was seen in 11 (37%) patients and in two (4%) controls (p less than 0.05). The difference in the mean permeability values between patients en controls was statistically significant (0.053 nm/sec. +/- 0.043 SD and 0.038 nm/sec. +/- 0.017 SD, respectively, p less than 0.05). No relation was found between the permeability values of the corneal epithelium and the NOSPECS classification or the duration of the disease. There was a tendency for increased permeability values in patients having an exophthalmos larger than 20 mm. (correl. coeff. = + 0.3, p less than 0.05).