Nearly one in 10 U.S. children is being diagnosed with attention deficit hyperactivity disorder (ADHD), according to a new analysis of federal data released Thursday.

The percentage of U.S. children between ages 5 to 17 who were diagnosed with ADHD increased from about 7 percent to 9 percent between 1998 and 2009, according to the analysis by the National Center for Health Statistics.

As expected, the condition was more common among boys than girls, according to the analysis. The prevalence of ADHD increased from 9.9 percent to 12.3 percent among boys and from 3.6 percent to 5.5 percent among girls.

The condition, which is marked by difficulty paying attention, impulsive behavior and hyperactivity, varies by race and ethnicity, according to the report. But the gap between whites and blacks narrowed during that time period, according to the report. The prevalence of the condition increased from 8.2 percent to 10.6 percent among whites compared to an increase from 5.1 percent to 9.5 percent among blacks.

Puerto Rican children had about the same rate as blacks, while the rate among Mexican Americans remained lower, according to the analysis.

The prevalence also varied by region of the country, with the rates being higher in the South and Midwest than the Northeast and West.

The analysis did not examine the reasons for the increasing diagnoses or the variation by race and region.