A "worryingly high" number of children with type 1 diabetes have warning signs of long-term health complications, including blindness, a report says.
And charities say this leaves "great concern" for their future health – some patients show early markers of eye, heart and kidney disease.
Data from England and Wales suggests more than 1,000 new cases were recorded in people under-25 last year.
But overall blood sugar control in this group has improved, the study shows.
Type 1 diabetes often appears in childhood. It is caused by the immune system destroying cells in the pancreas, leaving patients unable to control their blood sugar.
This is different to the far more common type 2 diabetes, which is largely due to an unhealthy lifestyle.
The Royal College of Paediatrics and Child Health (RCPCH) looked at figures from young people's diabetes units across England and Wales in 2013-14.
Its analysis shows that 26,867 children and young people with diabetes were recorded last year, compared with 25,221 in 2012/13.
But just 16% underwent all seven annual health checks that are recommended to monitor their blood sugar control and any complications.