Increased cupping of the optic disc is considered to be an indication of pressure-related damage of the optic nerve. This paper explores the relationship of intraocular pressure and cupping in persons with diabetes mellitus, a group of people whose optic nerves may be more susceptible to the effects of intraocular pressure. Stereoscopic fundus photographs of the seven standard fields were obtained in all persons participating in the Wisconsin Epidemiologic Study of Diabetic Retinopathy at the time of the initial prevalence survey. Measurements of disc and cup diameters in the vertical and horizontal meridia were made by two trained graders. Cup-to-disc ratios were computed for both diameters of each eye and the mean of the two gradings was used. A history of glaucoma was significantly associated with larger cup-to-disc ratios at the prevalence examination. Cup-to-disc ratios were not larger in those with high IOP, nor in those who had panretinal photocoagulation.