PurposeThe multicenter Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study is a prospective, observational study of 1,209 keratoconus patients. We report on the prevalence of corneal scarring in these patients. We also report on the test-retest repeatability of corneal scar documentation at the slit-lamp biomicroscope by trained clinicians and by masked photograph readers and on the scarring-status agreement at baseline between clinicians and photograph readers.MethodsClinicians and masked photograph readers graded each cornea as to scarring status. Patients were examined by clinicians, and their corneas were photographed at baseline (2,297 nongrafted eyes of 1,209 patients) and at a repeated visit (258 nongrafted eyes of 138 patients). These photographs were evaluated by the masked readers at the CLEK Photography Reading Center (CPRC). Clinicians reported corneal scars in right eyes at baseline as “definitely not present‘’ in 53.9%, “probably not present‘’ in 8.4%, “probably present‘’ in 8.2%, and “definitely present‘’ in 29.4% of patients. A weighted kappa statistic of 0.83 (95% confidence interval from 0.78 to 0.88) indicates that agreement is excellent between baseline and repeated assessments for the presence of a corneal scar by clinicians.ResultsAgreement is very good between baseline and repeated photograph-reader assessments for the presence of a scar, with a weighted kappa of 0.77 (95% confidence interval, 0.72–0.82). The kappa statistic comparing photograph-reader scarring assessments with clinician results was 0.69 (95% confidence interval, 0.66–0.71).ConclusionThe data also suggest better agreement between clinicians and readers when Vogt's striae and corneal nerves were observed. The data also suggest better agreement when corneal staining was not observed by the photograph readers. The CLEK Study protocol for determining the presence of scars is highly repeatable.