Dellen may occur in the corneas of eyes with filtering blebs. Nine cases of such dellen were encountered in a series of 97 consecutive trabeculectomies. In two such patients, the dell progressed to a deep corneal ulcer, although in most cases it healed uneventfully with minor changes in therapy. The use of topical prednisolone acetate during the postoperative period may also contribute to dellen formation by inhibiting corneal epithelial wound healing. However, the most important factor predisposing to dellen formation was poor tear-film integrity adjacent to the bleb. The secretion of the aqueous tears seemed to have little or no effect on dellen formation. Almost all dellen occurred in eyes with large filtering blebs.