To determine the influence of knowledge of the chronology with which stereosco pic photographs of the optic disc were obtained on how the photographs are inter preted. Case series. Disc photographs of patients enrolled in the Advanced Glauc oma Intervention Study and Collaborative Initial Glaucoma Treatment Study studie s from Wills Eye Hospital. Two sets of interpretable optic disc stereo slides we re included for evaluation. All patients had a follow-up of at least 5 years wi th baseline and follow-up photographs. Three observers (1 glaucoma attending ph ysician and 2 glaucoma fellows) evaluated the stereophotographs in chronological order to identify the clinical impression of glaucomatous change (glaucomatous deterioration or improvement) based on information such a s neuroretinal rim width, nerve fiber layer defects, rim color, peripapillary at rophy, disc hemorrhages, and blood vessel alterations. Three months later, the s lide pairs were shuffled so the observers were masked to the chronological seque nce. The photographs were reevaluated to determine if there was glaucomatous cha nge. Agreement between the 2 readings was measured for each observer individuall y and for the 3 observers collectively. The difference between the number of pho tographic pairs identified as worse when chronological order was known versus un known was evaluated using a chi-square test. Intraobserver and interobserver ag reement were assessed in terms of κvalues. Intraobserver agreement between chro nologically masked and unmasked readings was 61%, 64%, and 71%for the 3 obser vers, respectively. Average intraobserver agreement between masked and unmasked readings was 65%(κ, 0.35). Agreement in identifying change (either deteriorati on or improvement) was 59%, 70%, and 68%, respectively, for each observer. Ag reement in identifying stability was 63%, 62%, and 73%, respectively. The num ber of cases identified as having deteriorated was significantly higher (101 vs 54)-when the observer knew the chronological order with which the photographs w ere taken (P=0.007). Interobserver agreement was higher when the chronology of p hotographs was known (κ, 0.68±0.05 for unmasked evaluation; κ,0.30±0.05 for masked evaluation). When disc photographs are read with knowledge of the chronol ogy with which they were obtained, the observations differ considerably from whe n the readings are made without this knowledge. To determine the influence of knowledge of the chronology with which stereosco pic photographs of the optic disc were obtained on how the photographs are inter preted. Case series. Disc photographs of patients enrolled in the Advanced Glauc oma Intervention Study and Collaborative Initial Glaucoma Treatment Study Studie s from Wills Eye Hospital. Two sets of interpretable optic disc stereo slides we re included for evaluation. All patients had a follow-up of at least 5 years wi th baseline and follow-up photographs. Three observers (1 glaucoma attending ph ysician and 2 glaucoma fellows) evaluated the stereophotographs in chronological order to identify the clinical impression of glaucomatous change (glaucomatous deterioration or improvement) based on information such as neuroretinal rim width, nerve fiber layer defects, rim color, peripapillary at rophy, disc hemorrhages, and blood Three months later, the s lide pairs were shuffled so the observers were masked to the chronological seque nce. The photographs were reevaluated to determine if there was glaucomatous cha nge. Agreement between the 2 readings was measured for each observer individuall y and for the 3 observers collectively. The difference between the number of pho tographic pairs identified as worse when Intraobserver agreement between chi-nologically masked and unmasked readings was 61%, 64%, and 71% for the 3 Observations, respectively. Average intraobserver agreement between masked and unmasked readings was 65% (κ, 0.35). Agreement identifying changes (either deteriorati on or improvement) was 59%, 70%, and 68%, respectively, for each observer. Ag néement in identifying stability was 63%, 62%, and 73%, respectively. The num ber of cases identified as having deteriorated was significantly higher (101 vs 54) -when the obse rver knew the chronological order with which the photographs were erected (P = 0.007). Interobserver agreement was higher when the chronology of p hotographs was known (κ, 0.68 ± 0.05 for unmasked evaluation; κ, 0.30 ± 0.05 for masked evaluation). When disc photographs are read with knowledge of the chronology with which they were obtained, the observations differ considerably from whe n the readings are made without this knowledge.