Serial fundus photographs of 14 eyes that eventually developed glaucomatous visual field defects and 110 slides from 76 eyes of race- and age-matched controls were reviewed in randomized masked fashion. Each eye that lost visual field demonstrated consistent abnormalities of the nerve fiber layer, beginning as early as 5 years (mean, 1 1/2 years) before it developed glaucomatous visual field defects on routine Goldmann perimetry. Preliminary estimates, based on regression analysis of this small series, suggest that half of these eyes (median) might demonstrate such reproducible abnormalities between four and six years before onset of their visual field defects. Only 9% of the matched controls showed similar nerve fiber layer changes, and in the one instance where analysis was possible, these were inconsistent and nonreproducible. Nerve fiber layer assessment by means of fundus photographs may be the earliest, surest means of distinguishing ocular hypertension from true glaucoma.