This paper describes two subgroups of primary open angle glaucoma: senile sclerotic glaucoma in the elderly with relatively low intraocular pressures, normal chamber angles, senile excavation, peripapillary atrophy and choroidal sclerosis; high tension glaucoma in the younger age-group with high intraocular pressures, signs of mesodermal dysgenesis in the chamber angle, deep and steep excavation, little peripapillary atrophy and no choroidal sclerosis. The description of senile sclerotic glaucoma is new as are the differences in the chamber angle and peripapillary atrophy. The size of the visual field defects in the upper and lower half of the visual field is similar in high tension glaucoma. In senile sclerotic glaucoma the larger defect tends to be in the upper half of the visual field. It is suggested that in senile sclerotic glaucoma the primary cause of damage is local vascular disease, and that in high tension glaucoma the intraocular pressure with a secondary role for an insufficient blood supply is the major cause of damage. Primary open angle glaucoma can be characterized by the ratio of pressure risk factors and vascular risk factors.