ATLANTA - Decay in children's baby teeth is on the rise, a worrying trend that signals that the preschool crowd is eating too much sugar, according to the largest government study of the nation's dental health in more than 25 years.

But there was some good news: Older children have fewer cavities and adults have less periodontal disease than in the past, and more of the elderly are retaining their teeth.

"Overall, we can say that most Americans are noticing an improvement in their oral health," said the study's lead author, Bruce Dye of the National Center for Health Statistics.

Experts are concerned about the prevalence of cavities in children ages 2 to 5. It increased to 28 percent in 1999-2004, from 24 percent in 1988-94.

Tooth decay in young children had been decreasing for 40 years. Some studies have suggested the trend might have ended, but the new report contains the first statistically significant indication that the trend has reversed, dental experts said.

One reason is that parents are giving their children more processed snack foods than in the past, and more bottled water or other drinks instead of fluoridated tap water, Dye said.

"They're relying more on fruit snacks, juice boxes, candy and soda" for the sustenance of preschoolers, he said.

Other experts agree that diet is at least part of the explanation.

"The same things contributing to the obesity epidemic can also contribute to tooth decay," said Gary Rozier, a dentist who teaches public-health policy at the University of North Carolina.

Inadequate dental care may also play a role. Cavities in young children can form very quickly, and parents should begin bringing their children to the dentist at age 1, said Joel Berg, chairman of the University of Washington's department of pediatric dentistry.

Parents also must help their young children brush properly, Berg said.

"Preschoolers don't have the dexterity to really clean their teeth," he said.

Baby teeth naturally fall out as children age, but dentists say untreated decay can spread and is too dangerous to go untreated.

Baby teeth are treated with fillings or - if the decay is extensive - extraction. But baby teeth fill certain spaces in the mouth, so their early removal may lead to crowding when adult teeth come in.

The study is based on an annual federal survey of about 5,000 people. It includes detailed in-person health interviews, and medical and dental examinations by health-care professionals.

The study averaged the findings from surveys done in 1988-94 and compared them with the average results from surveys done in 1999-2004.

Experts were heartened that the study found that cavities in permanent teeth decreased to 21 percent of children in 1999-2004, from 25 percent in 1988-94.

That may be at least partly due to the growing prevalence of dental sealants, a plastic coating applied to teeth that protects against decay.