Objective: To determine the frequency with which visual field abnormalities observed on follow-up visual fields for patients in the Ocular Hypertension Treatment Study were confirmed on retest. Methods: Between April 1, 1994, and March 1, 1999, 21603 visual fields were obtained from 1637 patients in the Ocular Hypertension Treatment Study. When follow-up visual fields are outside the normal limits on the Glaucoma Hemifield Test, the Corrected Pattern Standard Deviation (P<.05), or both, subsequent follow-up visual fields are monitored to confirm the abnormality. Abnormalities are confirmed if they are again abnormal on the Glaucoma Hemifield Test, the Corrected Pattern Standard Deviation, or both; if the defect is not artifactual; and if the same index and location are involved. Reliability criteria used by the study consisted of a limit of 33% for false positives, false negatives, and fixation losses. Results: Of the 21603 regular follow-up visual fields, 1006 were follow-up retests performed because of an abnormality (n = 748) or unreliability (n = 258). We found that 703 (94%) of the 748 visual fields were abnormal and reliable, and 45 (6%) were abnormal and unreliable. On retesting, abnormalities were not confirmed for 604 (85.9%) of the 703 originally abnormal and reliable visual fields. Conclusions: Most visual field abnormalities in patients in the Ocular Hypertension Treatment Study were not verified on retest. Confirmation of visual field abnormalities is essential for distinguishing reproducible visual field loss from long-term variability.