Framingham Eye Study data were used to examine the effect of alternative definitions of open-angle glaucoma (OAG) on prevalence and on associations with selected variables from the Framingham Heart Study. Definitions were based on various combinations of history of glaucoma, types of visual field defects, and functions of intraocular pressure and cup-disc ratio. Visual field defect irrespective of blind spot enlargement is used as a standard for comparison. Prevalence of OAG by this definition is higher for men than for women and increases with age, more markedly than for most other definitions. Alcohol consumption is directly related to glaucoma defined either by the reference standard or by visual field defects including blind spot enlargement. Systemic blood pressure, ventricular rate, and diagnosis of diabetes are directly related to IOP.