OBJECTIVE To quantitatively evaluate the rate of cup-to-disc ratio progression in treated patients with primary open-angle glaucoma and to identify clinical factors associated with cup progression. METHODS Fifty one eyes of 51 treated primary open-angle glaucoma patients with a minimum of 9-year longitudinal series of stereoscopic optic disc photographs were studied. Eyes with any other ocular disease except for mild cataract were excluded. Each set of stereoscopic photographs was digitized and viewed stereoscopically on a computer screen using a hand-held stereoscope. Computer-aided planimetry was performed on each set of photographs with examiner-defined cup and disc margins using custom-made software. The software computed linear cup-to-disc ratios as well as peripapillary atrophy area. Both inter-observer and intra-observer reliabilities were evaluated in a masked, random fashion using intra-class correlation. Changes in linear cup-to-disc ratios and peripapillary atrophy were estimated using linear regression over time. All available clinical factors were evaluated for association with the rate of cup progression using a multiple regression model. RESULTS All patients studied were Caucasian; 31(61%) were females. The mean age at the beginning of the study was 61.6 +/- 7.8 years (range 46-81). The mean follow-up period was 14.3 +/- 3.5 years (median 14.0, range 9.6-22.3). A total of 173 sets of stereo disc photographs were analyzed (3.4 +/- 1.3 per patient). The initial and final linear cup-to-disc ratios were 0.64 +/- 0.15 and 0.73 +/- 0.14 respectively. The inter-rater and intra-rater intraclass correlations were 0.76 (95% CI 0.61-0.87) and 0.97 (95% CI 0.93-0.98) respectively. Using linear regression the rate of linear cup-to-disc ratio change was 0.0068 +/- 0.0062 per year (range -0.0025- 0.0269). Three eyes had an increase in the peripapillary atrophy area. The higher yearly average intraocular pressure was significantly associated with faster rate of linear cup-to-disc ratios progression (P = 0.03). CONCLUSIONS In treated patients with primary open-angle glaucoma, the rate of progressive optic disc cupping was slow (LCDR progression of 0.0068 per year). The higher yearly average intraocular pressure was significantly associated with a faster rate of cup progression.