The payoff for this "treatment" of children, 5 to 18 years old, who participated in a nationwide study, was a significant reduction of symptoms. The study appears in the September issue of the American Journal of Public Health.

"The advantage for green outdoor activities was observed among children living in different regions of the United States and among children living in a range of settings, from rural to large city environments," wrote co-authors Frances E. Kuo and Andrea Faber Taylor. "Overall, our findings indicate that exposure to ordinary natural settings in the course of common after-school and weekend activities may be widely effective in reducing attention deficit symptoms in children."

ADHD is a neurological disorder that affects some 2 million school-aged children, as well as up to 2 to 4 percent of adults, in the United States. Those with ADHD often face serious consequences, such as problems in school and relationships, depression, substance abuse and on-the-job difficulties.

"These findings are exciting," said Kuo, a professor in the departments of natural resources and environmental sciences and of psychology at Illinois.

"I think we're on the track of something really important, something that could affect a lot of lives in a substantial way," she said. "We're on the trail of a potential treatment for a disorder that afflicts one of every 14 children -- that's one or two kids in every classroom."

If clinical trials and additional research confirm the value of exposure to nature for ameliorating ADHD, daily doses of "green time" might supplement medications and behavioral approaches to ADHD, the authors suggest in their conclusion.

Kuo and Faber Taylor, a postdoctoral researcher who specializes in children's environments and behavior, recruited the parents of 322 boys and 84 girls, all diagnosed with ADHD, through ads in major newspapers and the Web site of Children and Adults with Attention Deficit/Hyperactivity Disorder.

The parents were interviewed by means of the Web and asked to report how their children performed after participating in a wide range of activities. Some activities were conducted inside, others in outdoor places without much greenery, such as parking lots and downtown areas, and others in relatively natural outdoor settings such as a tree-lined street, back yard or park.

The researchers found that symptoms were reduced most in green outdoor settings, even when the same activities were compared across different settings.

"In each of 56 different comparisons, green outdoor activities received more positive ratings than did activities taking place in other settings, and this difference was significant or marginally significant in 54 of the 56 analyses," Kuo said. "The findings are very consistent."

The two researchers have been pursuing the ADHD issue as an extension of a long line of previous research they've conducted on the nature-attention connection among the general population in mostly urban settings.

"The medications for ADHD that are currently available work for most kids, but not all," Kuo said. "They often have serious side effects. Who wants to give their growing child a drug that kills their appetite day after day and, night after night, makes it hard for them to get a decent night's rest? Not to mention the stigma and expense of medication."

Simply using nature, Kuo said, "may offer a way to help manage ADHD symptoms that is readily available, doesn't have any stigma associated with it, doesn't cost anything, and doesn't have any side effects -- except maybe splinters!"

There are a number of exciting possible ways in which "nature treatments" could supplement current treatments, she said.

Spending time in ordinary "urban nature" -- a tree-lined street, a green yard or neighborhood park -- may offer additional relief from ADHD symptoms when medications aren't quite enough. Some kids might be able to substitute a "green dose" for their afternoon medication, allowing them to get a good night's sleep.

"A green dose could be a lifesaver for the 10 percent of children whose symptoms don't respond to medication, who are just stuck with the symptoms," Kuo said. As Kuo and Faber Taylor wrote, a dose could be as simple as "a greener route for the walk to school, doing classwork or homework at a window with a relatively green view, or playing in a green yard or ball field at recess and after school."

The National Urban and Community Forestry Advisory Council, U.S. Forest Service, and the U.S. Department of Agriculture's Cooperative State Research, Education, and Extension Service supported the project.