Snoring can cause long term cardiovascular, neurocognitive and behaviour problems in children, a new study has found.

Most parents believe their child's snoring is harmless and something they will grow out of, but a Monash University study says it can be dangerous.

The study found school-aged children who snore had higher blood pressure, increased bad behaviour and a reduced intellectual ability.

"It's the first study which has actually shown this evidence of actual brain damage," Professor Rosemary Horne said.

"We think the longer the child snores, the worse the effects are going to be."

Prof Horne's team used MRI scans and measurements of oxygen levels in the brain of 136 children aged seven to 12 and 128 children aged three to five.

Children were then followed up three to four years later.

Preschool-aged children who snored were found to have normal blood pressure and neurocognitive development but had increased reports of poor behaviour, so early intervention is key.

Prof Horne said the most common cause of childhood snoring is enlarged tonsils and adenoids, and concerned parents should contact their GP.