The association between diabetes mellitus and primary glaucomas is evaluated. Diabetes is specifically associated with narrow-angle categories of primary glaucoma: closed-angle glaucoma and narrow-angle patients with ocular hypertension or open-angle glaucoma. The prevalence of diabetes mellitus in wide-angle glaucoma patients is not significantly different from normal age-matched control subjects. The association between diabetes and glaucoma is restricted to non-insulin dependent diabetes mellitus and impaired glucose tolerance; the prevalence of insulin-dependent diabetes mellitus in the primary glaucomas is equivalent to age-matched normal values. These conclusions are supported by 75-gram oral glucose tolerance testing, estimation of glycosylated haemoglobin, retrospective analysis of positive diabetic family history, and prospective follow-up examining progression of impaired glucose tolerance to diabetes mellitus in patients with primary glaucoma.