More teenagers are being diagnosed with kidney stones now than in years past, a study from one U.S. state suggests.

The research, which followed Minnesota children from 1984 to 2008, found that the rate of kidney stones climbed six percent each year among teenagers.

Between 1984 and 1990, the annual rate was 13 cases for every 100,000 12- to 17-year-olds. That figure nearly tripled, to 36 per 100,000, between 2003 and 2008.

Researchers say they are not sure of the reasons -- or even whether teenagers are actually suffering the stones at a higher rate. It may be that more cases are being diagnosed since doctors started using highly sensitive CT scans that can detect smaller kidney stones.

"There has been much speculation about how the incidence of kidney stones might be changing over time in the pediatric population," lead researcher Dr. Moira E. Dwyer, of the Mayo Clinic in Rochester, said in an email.