Doctors are puzzling over what seems to be an increase in the number of children with kidney stones, a condition some blame on kids' love of cheeseburgers, fries and other salty foods.

Kidney stones are usually an adult malady, one that is notorious for causing excruciating pain—pain worse than childbirth. But while the number of affected children isn't huge, kids with kidney stones have been turning up in rising numbers at hospitals around the country.

Kids' stones have been the talk of recent pediatric kidney specialists' conferences, said Dr. Uri Alon, director of the bone and mineral disorders clinic at Children's Mercy Hospital in Kansas City.

So far, the only evidence is anecdotal. But Alon is involved in research trying to determine if the increase is real and not just the result of greater awareness and better ways of detecting stones. Alon also is studying whether improved nutrition can prevent kids' kidney stones.

Eating too much salt can result in excess calcium in the urine. In children, most stones are calcium-based, and Alon said their eating habits, plus drinking too little water, puts them at risk. Plenty of water is generally recommended to help prevent kidney stones.

Matty Billemeyer is just 8 years old but already has had four bouts with stones, the first in 2007, the last a year ago in April. He was first stricken in his first-grade class; the school nurse, his parents and even the emergency room doctors all thought it was his appendix.

Darryl Billemeyer said it was frightening seeing his son writhing and screaming in pain. The boy was transferred from a local hospital to Children's Hospital of Philadelphia, where ultrasound tests showed kidney stones.

The first time, Matty needed surgery; the other times the stones passed during urination.

Now he takes diuretic pills to increase urination, brings a water bottle to school everyday, and has given up favorite foods, including sausages, pickles and packaged ramen noodles—all high in salt.

The main problem associated with kidney stones is extreme pain. It is caused by stones blocking urine flow, which, if untreated, could lead to kidney damage.

The preferred treatment is observation—giving kids pain medicine but nothing else to see if the stones will pass on their own. Stones can be as small as a sugar granule or as large as a pearl. Bigger ones have been reported but are rare; most are less than 1/4 inch in diameter, which can usually pass on their own. But even small ones can mean incredible pain.

Dr. Barry Duel, a pediatric urologist at Cedars-Sinai Medical Center in Los Angeles, said kidney stones can be a sign of underlying metabolic problems that result in too much calcium in the urine. But he said in most cases children have no underlying disorder and are otherwise healthy. Still, because some metabolic problems can slow growth if untreated or lead to repeated bouts with kidney stones, the American Academy of Pediatrics recommends metabolic testing for all children with kidney stones.