Despite recent improvements in the management of blood glucose control
and in modern technology such as laser treatment and vitreoretinal su
rgery, diabetes mellitus is the major systemic cause of blindness in t
he Western world. The study of the natural history of diabetic retinop
athy is difficult because of the variability of the disease, and the n
umerous factors that may influence its course and outcome. In the many
studies available of the natural history of diabetic retinopathy, the
re is a failure to identify these factors. This article tries to ident
ify and to classify the influences that may modify the outcome and the
natural course of diabetic retinopathy, and gives some advice on how
to deal with them. They can be subdivided into external, internal and
ocular factors. Any future studies of the natural history of diabetic
retinopathy are marred by the fact that effective treatment is now est
ablished and the guidelines for when to commence treatment and the tec
hniques of treatment are well documented.