PHILADELPHIA - Some parents are getting letters home from school these days, but not because their youngsters are acting up or flunking out. The problem is their children are too fat.

The letters are worded with more sensitivity than that, of course, but the idea is to encourage parents to change their children's eating habits and help them get more exercise.

Parents of students in the East Penn school system in Pennsylvania and in Florida's Citrus County district have been getting such weight alerts since the fall.

''When an examination reveals a child has vision problems, hearing problems, we inform the family. We weren't doing anything for weight,'' said George Ziolkowski, director of pupil personnel services for the 6,800-student East Penn district, about 60 miles northwest of Philadelphia. ''If we have information that may have some bearing on a child's future health, why just put it in a drawer?''

Stephanie Hertzog said the letter she received about her 12-year-old son Michael, a fifth-grader, came as no surprise to either of them.

"He's a very husky boy. We know about his weight problem," she said, adding that the notification helped her broach the subject with her son.

About 380 confidential letters have gone out to elementary and middle school students since the program began in the fall, with more expected once high school screenings are completed, Ziolkowski said.

He said it took months to craft the letter, and school nurses and health educators worked to ensure it contained neutral language that would not make parents feel they were being blamed.

Similarly, the letter sent in Citrus County assures parents that the purpose is to ''assist in appraising, protecting and promoting the health status of your child. It is intended to encourage good nutritional habits and healthy physical activity.''

The Centers for Disease Control and Prevention says the percentage of children and adolescents who are defined as overweight has more than doubled since the early 1970s, constituting a public health epidemic.

Nationwide, about 13 percent of children and adolescents are overweight or obese, as are nearly 60 percent of adults, according to a report released in December by then-Surgeon General David Satcher. Being overweight can contribute to diabetes and high blood pressure.

Computer games and television are among the reasons given for the problem, along with busy parents who rely on fast food for their kids.

Florida has mandated that the state Health Department gather data on student health and offer counseling to overweight youngsters. State health officials left it up to local health departments on how to do that.

In Citrus County, about 65 miles north of Tampa, the 15,000-student district and the county health department send every parent the results of a body-mass test, with a box checked indicating whether the child is normal, overweight, at risk of being overweight, or underweight. The letters alert parents of overweight children to the health risks.

Some parents complained that the school system was only calling attention to their child's weight problem, ''when the fact of the matter is that they're reminded of that fact every day'' by classmates, said Sylvia Byrd of the Florida Health Department's School Health Program.

''To people who say this hurts their self-image, I would say that the biggest boost for their self-image would be to get them to a healthier weight and a more active lifestyle.''

Dr. Andrew Tershakovec at Children's Hospital of Philadelphia said the letters are a good idea. ''It's wholly appropriate for the public health community to identify children in this way,'' he said. ''It's a public health issue.''