BARNSLEY children have the highest levels of tooth decay in South Yorkshire, with 41 per cent affected.
A major study will be carried out in October after figures revealed a high number of five-year-olds showed signs of obvious tooth decay.
Public Health Barnsley will now conduct a study of thousands of children across the town's 79 primary schools to see how widespread the problem is.
Barnsley is ranked 22 out of 147 (one being the worst), despite 75 per cent of children visiting a dentist regularly.
Each child experiencing tooth decay had on average three or four affected baby teeth.
Dr Frances Clement, a consultant in public health medicine, said she was shocked at the results.
"Given that we are doing as much as we can, we thought we would have had better statistics. To be the worst in South Yorkshire isn't very good at all.
"We weren't happy with the result and thought it should have been better given all the efforts we've been putting in. It is not getting worse, but it's not getting better either."
Dr Clement said reasons for the tooth decay could include 'parental behaviour'.
She said giving children sweets when they are playing up could cause problems, and that eating and drinking things which are perceived to be healthy, such as fruit juices which contain lots of sugar, can lead to plaque.
