The study also noted a drop in the proportion of non-elderly adults who have visited a dentist in the past year - a possible indicator of declining dental insurance.
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, Dr. Bruce Dye of the National Center for Health Statistics.
Experts are concerned about the prevalence of cavities in baby teeth of children ages 2 to 5. It increased to 28 percent in 1999-2004, from 24 percent in 1988-1994, according to the report.
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 proof 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.
Others experts agree diet is at least part of the explanation for the rising cavity rates.
"The same things contributing to the obesity epidemic can also contribute to tooth decay," said Dr. 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 Dr. Joel Berg, chairman of the University of Washington's Department of Pediatric Dentistry.
Parents also must help their young children brush properly. "Preschoolers don't have the dexterity to really clean their teeth," Berg said. Baby teeth naturally fall out as children age, but dentists say untreated decay can spread and is too dangerous to go untreated. Rotten 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 1988-1994 and compared them with the average results from surveys done in 1999-2004. The results are being reported Monday at a meeting of the American Association for Public Health Dentistry in Denver. 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 1988-1994. That may be at least partly due to the growing prevalence of dental sealants, a plastic coating applied to teeth that protects against decay. About 38 percent of children and teens ages 12 to 19 had dental sealants in the most recent set of surveys. Some of the other findings: - Among senior citizens ages 65 and older, the percentage with complete tooth loss dropped to 27 percent, from 34 percent. - Moderate and severe gum disease in adults ages 20 to 64 dropped to 5 percent, from 10 percent. Gum disease dropped to 17 percent, from 27 percent, in seniors. - Tooth decay in the permanent teeth of children ages 6 to 11 dropped to 21 percent, from 25 percent. Tooth decay in youths ages 12 to 19 dropped to 59 percent, from 68 percent. - The percentage of adults who said they'd been to a dentist in the previous year dropped to 60 percent, from 66 percent. AP-CS-04-30-07 0930EDT