There is growing concern about increasing rates of diagnosis of
autism and attention-deficit hyperactivity disorder (ADHD) and
about whether this observed increase is associated with
environmental agents (1,2,3), improved ascertainment (4,5), or
overdiagnosis (6,7,8,9). Although recent cross-sectional studies
have examined the prevalence of ADHD (10,11) and autism
(12,13,14), few studies have examined trends in the prevalence
of these disorders. Some studies have examined trends in treated
prevalence. For example, Kelleher and colleagues (5) found that
pediatricians identified attention problems such as ADHD among
1.4 percent of children in 1979 and 9.2 percent of children in
1996, an increase of 657 percent. Data from the U.S. National
Ambulatory Medical Care Survey suggest a 250 percent increase
between 1990 and 1998 in the number of children receiving a
diagnosis of ADHD (15). Zito and colleagues (16) found that
children's ambulatory care discharges for ADHD increased from
1.9 percent to 3.6 percent. Simil