Only a third of children in Wales have healthy teeth overall and this is lagging behind England, a major survey suggests.
Thousands of children have been examined every 10 years since 1973.
A third of children in Wales were said to have good oral health overall, which compares to 39% in England.
Decay was found in 52% of 12 year olds and 63% of 15 year olds, a slight improvement on the 2003 survey.
But around 70% of them reported problems with their teeth.
Decay was found in 41% of five-year-old children in their primary teeth and in 55% of eight-year-old pupils, according to the Health and Social Care Information Centre report.
Severe dental problems were worse among children from poorer backgrounds.
Around a half (47%) of five year olds could be said to have good oral health.
But that declined with age, to around a quarter of 15 year old children.
- There is "no evidence of change in the proportion of older children with tooth decay between 2003 and 2013".
- Despite many children reporting positive views of their own dental health, dental problems are common.
- The most common problem was sensitive teeth, followed by mouth ulcers, bad breath, toothache and bleeding gums.
- Toothache was more common in children from relatively deprived families.
- Over half of 12 and 15 year olds reported that their daily lives had been affected by problems with their teeth.
- Tooth brushing twice a day has increased significantly in eight year olds in Wales since the previous survey in 2003.
The last results in 2003 showed that oral health had improved greatly since the 1970s and 1980s.
This time, 12 and 15-year-olds were also asked to fill in a questionnaire on how they looked after their teeth.
In 2009 the Welsh government launched a national scheme to encourage better dental health amongst young children. Since then it has invested over £12m in the programme.
It also suggested the programme had led to a recent 6% drop in the proportion of five-year-olds experiencing dental decay.
Experts point out there could be several reason the health of children's teeth in Wales was still lagging behind those from other UK nations including socio-economic conditions.
That survey 10 years ago also showed a big drop in 15-year-olds with obvious decay from 94% in 1983 to 58%.
Scottish children are not included in the 2013 survey but Wales has better results than Northern Ireland, where 31% were judged to be in good oral health.
The Welsh Government said its own most recent monitoring survey showed a 6% drop in the proportion of five-year-olds with dental decay.
"However, there is more to do to improve the dental health of children in Wales especially since the vast majority of tooth decay is avoidable," said a spokesperson.
"Parents can help their children by reducing sugar intake and encouraging them to brush their teeth twice a day with fluoride toothpaste. "
It has also published its own survey of dental health to identify how much decay among five-years-olds is already present at the age of three.