We studied the influence of optic disc diameter on the diagnostic power of subjective disc evaluation of glaucoma. One hundred and thirteen colour disc photographs of 91 patients with primary open angle glaucoma and 174 photographs of 89 randomly selected normal subjects were studied. Five ophthalmologists independently classified all eyes as glaucomatous or normal by disc appearance using a masked forced choice design. Sensitivities and specificities varied strongly as a function of disc diameter. Overall sensitivity for recognizing glaucoma by disc inspection was 58% in small discs, 72% in the middle group and 85% in the group of large discs. These differences were significant, and the odds of recognizing a glaucomatous disc as such increased by a factor of 2.07 between each group. Overall specificity was 99.6% in small discs, 95% in the middle group, and 79% in large normal discs. Thus, larger discs were more likely to be classified as glaucomatous whether they were glaucomatous or not, while small discs were more likely to be classified as normal.