PURPOSE: To assess the reproducibility of determining whether an eye has developed optic disk deterioration by the Optic Disc Reading Center (ODRC) in the Ocular Hypertension Treatment Study (OHTS). DESIGN: Test-retest reproducibility study. METHODS: Masked, certified graders at the ODRC determine the occurrence of optic disk deterioration in OHTS by comparing baseline with follow-up stereoscopic optic disk photographs. To assess reproducibility, regradings were obtained annually by inserting masked "quality control" photographs into the usual ODRC reading stream. RESULTS: Agreement (kappa) ranged from 0.65 to 0.83 over 5 years. Specificity ranged from 98% to 100%, and sensitivity ranged from 64% to 81%. CONCLUSIONS: The kappa statistic for test-retest agreement in OHTS is in the range considered good to excellent over 5 years. Consistency (specificity) in regrading optic disks that did not develop deterioration was particularly high. The sensitivity results show that detecting subtle deterioration of optic disks is challenging.