Sixteen preglaucomatous eyes in which glaucomatous field defects developed were compared with 38 preglaucomatous eyes matched for age and sex whose fields remained normal. A noticeable enlargement of the cup occurred in 31% of those in which field loss developed. The cups of those in whom visual field defects developed were larger than in the controls, thus suggesting early acquired tissue damage preceding field loss. Field defects developed without noticeable enlargement and with a small initial cup in 12% of those observed, and these would not have been suspected from the initial disc appearance.