1. A grading method for new vessels arising from the optic disc is described. The grading system has been used to study the natural history of new vessels in forty-four eyes of thirty diabetic patients.
2. While some eyes showed considerable changes, the mean progression of new vessels in the entire group was slight and only reached one grade in four years.
3. The most striking feature of the study was the number of eyes which could not be fully assessed at different times during the follow-up period. Eyes became unassessable because of vitreous hemorrhage or retinitis proliferans. These increased from 18 per cent at six months to 66 per cent at four years. Both of these were more common when higher grading values of new vessels were reached.
4. The visual acuity deteriorated in twenty-five eyes. In nineteen of the eyes the deterioration was three lines or more; fifteen eyes became blind. The commonest cause for visual deterioration was vitreous hemorrhage.