Nearly 1,000 people in Braintree and Witham are thought to have undiagnosed Type 2 diabetes.
Charity Diabetes UK, which has offices in Witham, said there are nearly 16,000 poeple already living with diabetes in mid Essex overall.
It is urging people to be checked to find out if they are at high risk.
Left untreated, there is greater risk of complications such as amputation, blindness, kidney failure and stroke.
Almost half of people with Type 2 diabetes already have signs of complications by the time they are diagnosed, said the charity.
For information visit www.diabetes.org.uk/riskscore.