To the Editor. —The superior limbic filamentous keratitis as described by Theodore has, to date, no specific etiology. An interesting finding on three or four patients with this condition has been a markedly elevated protein-bound iodine value, and all patients have subsequently been found to be completely asymptomatic of hyperthyroidism. The fourth patient was on a dietetic regimen taking from 0.260 to 0.390 gm of thyroid extract per day. I think this is a most interesting finding in this disease and whether or not it has some connection with the etiology has yet to be determined. The main purpose of this correspondence is to bring this finding to the attention of other ophthalmologists around the country so that a larger series of these patients might be examined specifically for hyperthyroidism to see if this finding holds true.