PURPOSE:To investigate intrastromal corneal ring segment depth with a high-speed corneal optical coherence tomography (OCT) system. SETTING: Doheny Eye Institute, University of Southern California, Los Angeles, California, USA. METHODS:A prospective observational case series comprised 4 eyes of 4 patients receiving Intacs intrastromal corneal ring segments (Addition Technology, Inc.) for keratoconus. Optical coherence tomography (OCT) was performed between 7 days and 43 days after implantation. RESULTS:The slitlamp impression of intrastromal corneal ring segment implantation depth did not correlate well with OCT measurements (r 2 Z 0.68). The fractional implantation depth was correlated with several surgical variables using a stepwise multivariate regression model, and 2 statistically significant correlations were found. The position of the distal portions of the ring segments was shallower than that of the portion closer to the insertion site (P Z .003). Segments placed in the inferior cornea (P Z .008) experienced more distal shallowing. Shallower depth was associated with greater fractional anterior stromal compression (P Z .04). CONCLUSIONS:Shallower placement of intrastromal corneal ring segments may result in more complications, such as epithelial-stromal breakdown and extrusion, because of the greater anterior stromal tensile strain. The distal and inferior portions of intrastromal corneal ring segments tended to be placed at a shallower depth. Optical coherence tomography provided precise measurement of ring segment depth and may help identify implants that pose a greater risk for depth-related complications.Keratoconus is a progressive disease that causes thinning and ectasia of the corneal stroma, leading to decreased visual acuity from irregular astigmatism. Traditionally, keratoconic eyes are corrected with spectacles and rigid gaspermeable (RGP) contact lenses when possible. When these conservative measures prove intolerable or unsatisfactory, penetrating keratoplasty (PKP) is considered. Penetrating keratoplasty for keratoconus provides good visual results in most cases, 1 and complication rates have decreased over the past decade. 2 However, immune rejection and long-term endothelial failure remain significant risks. 3 In rare cases, keratoconus may recur 15 to 20 years later in the transplanted cornea. 4