Patients with type 1 diabetes develop an absolute requirement for insulin.

The peak age of onset is around 12 years.

There are marked differences in the prevalence of the disease, for example there is a 35-fold difference between Japan (low) and Finland (high). The reasons for this variability are unknown but environmental (e.g. diet, viruses) and genetic factors may underlie it.

In the UK the prevalence is roughly 0.25%, accounting for about 20% of cases of diabetes.

Type 1 diabetes presents more commonly in the autumn and winter: intercurrent infections may in part explain this seasonal variation.