Purpose: To evaluate the effect of an intraoperative topical intrastromal steroid on the incidence and severity of nonspecific diffuse intralamellar keratitis (NSDIK). Setting: T.L.C. The Laser Center, Brea, California, USA. Methods: This prospective study included 2 cohort groups, each consisting of 105 consecutive eyes treated with laser in situ keratomileusis (LASIK) for myopia and astigmatism. Group A received the standard LASIK medication protocol and Group B, an additional drop of prednisolone sodium phosphate 1% solution on the undersurface of the cap immediately after it was reflected, prior to initiating the laser treatment. After the laser treatment was completed, the flap was repositioned; 1 minute after the steroid application, the interface was irrigated. All patients were examined postoperatively and placed in 1 of 4 categories: no interface keratitis, grade 1 (mild), grade 2 (moderate with decreased vision), or grade 3 (severe with decreased vision). Results: The incidence of NSDIK in Group A was 17.1% (18 eyes); 14 eyes (78.0%) were grade 1 and 4 (22.0%), grade 2. The incidence in Group B was 6.7% (7 eyes), all eyes were grade 1. Topical intrastromal steroid application significantly reduced the incidence and severity of NSDIK (P < .01). Conclusion: We propose that intraoperative intrastromal steroid application is a safe and effective way to reduce the incidence and severity of NSDIK. J Cataract Refract Surg 1999;25:1437-1440 (C) 1999 ASCRS and ESCRS.