Twenty corneas from patients with interstitial keratitis were examined by light microscopy and one of these by electron microscopy. Seventy percent of patients had either a positive serologic test for syphilis or a history of treatment for syphilis. Focal or diffuse multilaminar thickening of Descemet's membrane with secondary linear guttata was present in 88% of cases. Confluent linear cornea guttata formed retrocorneal hyaline ridges that clinically showed a central gray core surrounded by a translucent sheath. Microscopically, these ridges consisted of concentric laminations of newly formed Descemet's membrane. Some ridges hung into the anterior chamber as a bow-like strand or a spiderweb network. A possible pathogenetic sequence may include (1) inflammatory insult to a relatively young endothelium; (2) alteration of endothelial function to fibroblast-like activity; (3) the production of abnormal basement membrane and other collagenous material and formation of a new multilaminar Descemet's membrane; and (4) separation of some ridges from the multilaminar Descemet's membrane to hang into the anterior chamber as strands and networks.