In 375 patients aged 68.5 +/- 9.4 years (mean +/- 1 SD) with primary glaucoma subdivided by irido-corneal angle configuration into closed-angle glaucoma (123 cases), ocular hypertension (186 cases), and open-angle glaucoma (66 cases), diabetic status was determined by standard 75 g oral glucose tolerance tests. In 63 patients with impaired glucose tolerance who were retested one year later, 16% had progressed to diabetes mellitus. The results confirm a significant association between non-insulin dependent diabetes mellitus and primary glaucoma which may result from autonomic dysfunction in parasympathetic tone causing anterior chamber angle closure.