As part of a human trial of phototherapeutic keratectomy, we performed anterior keratectomy using excimer laser 193-nm ablation on patients with superficial corneal opacities. We examined the ultrastructural changes in the corneas of four patients who underwent penetrating keratoplasty 6 to 15 months after excimer laser ablation. The four cases included macular dystrophy, recurrent keratoconus in a graft, and corneal scarring (two eyes). Light microscopy showed epithelial thickening, absence of Bowman's layer, and superficial stromal scarring in the area of ablation. Ultrastructural study showed that the epithelial basement membrane had focal discontinuities. At the margin of the ablation superficial collagen bundles terminated abruptly in a steplike configuration. The anterior stroma was scarred, with loss of lamellar structure and an increase in number of fibrocytes. The laser-induced scar was 10 to 15 microns thick in the two eyes without a stromal scar before laser ablation. The stroma underlying the scarred areas and that in the untreated area appeared normal. Linear collagen-like fibers were present in the posterior aspect of Descemet's membrane. Laser-induced scarring may be an important factor in limiting visual improvement in patients undergoing phototherapeutic keratectomy.