One in 50 people in Wales has undiagnosed diabetes, according to campaigners who are launching an effort to identify them.
About 150,000 people know they have the condition, but about another 66,000 are undiagnosed and risk developing serious complications.
The charity Diabetes UK Cymru wants more done to raise awareness.
The Welsh Assembly Government said it is focused on ensuring patients have access to high quality care.
The charity warns that if untreated, Type 2, the most common form of diabetes, can lead to strokes and amputations while among children, Type 1 can result in coma or even death.
People can have the condition for up to 10 years without knowing, meaning that half will have already developed complications, such as heart disease, strokes, kidney disease, blindness and amputations.
Type 1 is usually diagnosed under the age of 40, and the symptoms develop quickly over a matter of weeks. If left untreated, it can lead to diabetic ketoacidosis - dangerously high blood glucose levels that can lead to coma or death.
Diabetes Cymru UK has sent bilingual posters to doctors' surgeries and pharmacies throughout Wales spelling out the condition's symptoms.
They recommend anyone with the symptoms to seek a diabetes test.
Dai Williams, the charity's national director, said: "It is terrifying that one in 50 people in Wales are living with diabetes without knowing it, as untreated diabetes can have devastating consequences for their health.
"Diabetes UK Cymru has launched this campaign as the symptoms, such as feeling very tired, thirsty and going to the loo more often to urinate, can be hard to spot.
"Our poster has been designed to spell out the symptoms very clearly and I would urge everyone to familiarise themselves with them.
"The sooner the condition is diagnosed, the sooner it can be controlled and the risk of developing its complications reduced."
Sara Fôn Evans, 22, an assistant management information systems officer at Coleg Menai, from Bryngwran, Holyhead, was diagnosed with Type 1 diabetes at 13.
She was diagnosed after feeling constantly thirsty and losing a stone and a half in weight in weeks, both classic symptoms.
Her father Vaughan, who has Type 1 diabetes, tested her blood glucose levels and found they more than four times higher than they should have been.
"It was a big shock," she said. "My parents called up a family friend who was a doctor and he told them to take me straight to hospital.
"I did my first insulin injection by myself the morning after."
From then on she treated the condition with four insulin injections every day.
In January she started using an insulin pump, a device the size of a mobile phone which replaces insulin injections by constantly infusing fast-acting insulin into the body.
"A lot of people don't know the symptoms so it is good that we can make them aware of them and what diabetes is."
An assembly government spokesperson said: "The Welsh Assembly Government's focus is to ensure that people with, or at risk from, diabetes have access to high quality care.
"The implementation of the national service framework for diabetes and the work being undertaken right across Wales as part of our Chronic Conditions Management (CCM) policy is aimed at doing precisely that.