These new studies contradict earlier findings about girls with ADHD. But Hinshaw's explanation is simple: Unlike the six-to-12-year-old girls in his studies, girls in several previous studies were taking ADHD medication. Hinshaw's work also included a much larger sample than nearly all earlier studies and was conducted over a longer period of time.

"These girls, compared to a matched comparison group, are very impaired, academically and socially," said Hinshaw, an expert in child clinical psychology and developmental psychopathology, who continues to study them.

Hinshaw said the girls are rejected by their peers and have a harder time making and keeping friends. "Social problems with peers are quite predictive of long-term adjustment problems," he explained, "so it will be essential to observe outcomes as the sample matures."

The girls also were more likely to have had early experiences such as being adopted and histories of language problems and learning difficulties.

Hinshaw's study involved one of the largest samples in the world of preadolescent girls with ADHD. A total of 228 girls - 140 diagnosed with ADHD and 88 not diagnosed with ADHD - were studied intensively at six-week summer camps held three years in a row. There were approximately 80 girls at each year's camp, which ran in 1997, 1998 and 1999.

The girls were recruited to participate in the camps in a number of ways. For the ADHD sample, Hinshaw sent mailings to health maintenance organizations, clinics, hospitals, mental health centers, pediatric practices and local school districts. In addition, presentations were made to self-help groups, and advertisements were placed in newspapers.

For the comparison girls, similar mailings were sent to school districts and community centers in the San Francisco Bay Area, and advertisements were placed in papers. The ads were almost identical to the ads for girls with ADHD, except the wording emphasized "summer enrichment programs" rather than "summer enrichment programs for girls with attentional problems."

The girls who appeared to be good candidates for the program participated in several levels of family screenings and evaluations to make sure they met appropriate criteria for the study. In addition, the families of the girls with ADHD had to agree to take the children off of ADHD medication during the six weeks so that their natural behavior patterns could be observed..

The sample was ethnically diverse - 53 percent Caucasian, 27 percent African American, 11 percent Latina and 9 percent Asian American. Incomes of these families ranged from upper class to those receiving public assistance. Girls with IQs lower than 70, overt neurological damage, psychosis and medical conditions that precluded participation in camp activities were excluded from the study.

The girls spent six weeks enjoying the same activities that children who go to other summer camps enjoy, but they were very closely monitored by people who had training in micro-observation.

Their "counselors" took copious notes relating to each girl's activities; the staff was not aware of which girls had ADHD diagnoses.

The summer programs were located on the campus of a local school and featured a structured series of classroom, art, drama and outdoor activities. In addition, all the girls received individual neuropsychological assessments.

Hinshaw said that, during outdoor sports and play at camp, "the girls with ADHD were less likely to follow the directions of the teacher than the comparison girls. They were also more likely to tease their peers and show aggressive behavior, though not at the same rate as boys with ADHD in previous summer camps. They were also more likely to display social isolation-wandering and failing to become engaged in activities.

"As a group, these girls show as much executive function deficit on neuropsychological tests as boys who have been diagnosed with ADHD."

Hinshaw said that selected girls from the study sample are participating in brain imaging studies at UC Berkeley to better pinpoint both working memory and "executive function," through examination of brain-behavior relationships. "Executive function" refers to actions such as goal setting, planning, organization, monitoring one's behavior during an activity and changing strategies in response to alterations in a situation.

"These functions are crucial for long-term academic, social and occupational success," Hinshaw said. "Deficits in executive functions are seen in other disorders, such as autism, but they may well be the core underlying problems for youth and adults with ADHD."

The girls and their families currently are participating in a follow-up study, so some of them have been followed for five years into adolescence.

Although boys diagnosed with ADHD outnumber girls approximately three to one, said Hinshaw, it may be that some girls have been underdiagnosed, particularly those with the "inattentive type" of the disorder, which seems more prevalent in girls.

"The inattentive type of ADHD is marked less by disruptive, impulsive behavior and more by disorganized, unfocused performance," Hinshaw said. "The latter isn't as likely to be recognized or cause as much concern to teachers."

Hinshaw hopes his project will bring attention to a population of young girls whose problems may have been ignored.

"Our hope," he said, "is that these efforts will spur the field towards theoretically rigorous attempts to understand the underlying processes and mechanisms responsible for ADHD in both boys and girls and to provide a sound scientific basis towards better classification, prediction and intervention."