Nearly three quarters of children with suspected autism are waiting too long to be diagnosed, NHS figures for North Yorkshire have shown.
Guidelines by the National Institute for Health and Clinical Excellence say all children should be assessed for autism within three months of referral.
But of 254 children in North Yorkshire currently listed for assessment, 180 have exceeded that waiting period.
The NHS in North Yorkshire said "good progress" was being made to cut delays.
Scarborough Hospital is the worst performer in the county, figures from North Yorkshire's four Clinical Commissioning Groups (CCGs) show.
All children currently on the hospital's waiting list are due to exceed the recommended three month maximum by the time they are assessed.
Samantha Spivey, from Scarborough, whose six-year-old son, Freddie, has been waiting for an autism diagnosis since February, called the delay "very upsetting".
"You want to do the best for your child and you feel hopeless that you can't do it. But if you don't get the right diagnosis, the right mechanisms and the right support then he's not going to blossom.
"It's not just that he gets the label, it's the support that comes from it - the extra teams that could come and help him."
Tracey Hampton, from support group Scarborough Families and Friends of Autistic Spectrum Together, said the delays were "unacceptable".
Janet Probert, from the NHS body which took over from North Yorkshire and York Primary Care Trust (PCT) in April, said the demand for autism assessments had exceeded that initially calculated by the former PCT.
Mrs Probert, who leads the team responsible for coordinating autism services in North Yorkshire, said some families had been given the option of having their children assessed outside the region in a bidt cut delays.
"We are working closely with the four CCGs in North Yorkshire and York, who are responsible for commissioning autism assessment services, to create extra capacity for assessments which will help reduce waiting times.
"Good progress is being made with this, but it will take time."