One of the main reasons for critics to believe ADHD is over-diagnosed is because of the increase in medications being prescribed to treat ADHD. According to an article, Almost Half of Kids with ADHD Are Not Being Treated, Study Finds, on ScienceDaily.com, the amount of children taking medications for ADHD tripled between 1975 and 1987. The number of adolescents taking medication for ADHD more than doubled between 1990 and 1995. Indeed, throughout the years, the use of ADHD medications has increased dramatically.
But does that indicate children are being diagnosed with ADHD unnecessarily? Or could there be other reasons for this increase? A number of different events occurred between the year 1975 and 1987. ADHD was renamed from Minimal Brain Dysfunction to Attention Deficit Hyperactivity Disorder. The Feingold Association (founded 1976) received publicity for their additive free diet, originally created for children with allergies, but publicized as a way to decrease hyperactivity in children. Parents and doctors began to be more aware of ADHD and see it as a viable diagnosis.
During the years between 1990 and 1995, even more information about ADHD became generally available. CHADD (Children and Adults with ADD) came into existence in 1987 and ADDA (Attention Deficit Disorder Association) began in 1989, helping to bring awareness about ADHD to parents across the country. During this time, it was accepted that a person did not "grow out" of ADHD but often continued to suffer from symptoms into adulthood. ADHD was seen often on television, with regular debates over medication usage. Although this was negative publicity, it continued to improve awareness of ADHD. Doctors continued to become more aware and more knowledgeable about ADHD and the diagnosis and treatment of ADHD. Additional medications, besides Ritalin came onto the market.
As awareness of ADHD grew, so did the number of people seeking treatment for ADHD. As physician's confidence in the diagnostic process improved, family doctors were able to diagnose and treat ADHD. Access to treatment became more widespread and therefore increased.
Even with the increase in the number of people aware of ADHD and seeking treatment, research shows that ADHD is under-diagnosed, not over-diagnosed. Researchers at Washington University School of Medicine in St. Louis found a large percentage of children, 32% of boys and 55% of girls, who met the criteria for a diagnosis of ADHD, were not receiving any type of treatment. The author of the study, Wendy Reich, PhD, stated, "From a clinical point of view, this study affirms that for whatever reason, many children who could benefit from treatment are not receiving it."
Another interesting finding of the study was the amount of children without ADHD that were receiving treatment. The researchers found this amount to be very small, approximately 3% of boys and 3% of girls were receiving treatment without enough symptoms to have a diagnosis of ADHD. The children receiving treatment did exhibit some symptoms of ADHD, researchers indicated, but not enough for a formal diagnosis. In addition, all of the children had siblings with ADHD. The parents, therefore, had a reason for discussing ADHD with the doctor. Treatment was not sought out as a replacement for discipline.
Another article, "Is ADHD Being Overdiagnosed?", explains that the American Medical Association reviewed 20 years of studies and found no indication or evidence of overdiagnosing ADHD or overprescribing stimulant medications. 
See All of This Series:
Is ADHD Overdiagnosed?
Can ADHD Be Accurately Diagnosed?
Are We Overmedicating Our Children?
Pharmaceutical Companies Making a Profit on ADHD
Do Parents Seek a Diagnosis of ADHD instead of Disciplining Children?
ADHD Understanding the Problem, Updated 2008, March 24, TelosNet
"About Us - Feingold Association", 2008, Feingold Association of the United States
 "Almost Half of Kids With ADHD Are Not Being Treated, Study Finds", 2006, Aug 6, Washington University School of Medicine
 "Is ADHD Being Overdiagnosed", 1998, Nov 16, Meg Kissinger, Journal Sentinel
"ADHD Controversy", Date Unknown, Guy, FDU
"Dramatic Rise in ADHD Sparks Controversy" 2000, Dec 15, Diane Weaver Dunne, Education World
"ADHD", Reviewed 2005, Jan 30, Ronald Pies, WebMD
"Myths and Misunderstandings", 2007, Author Unknown, Help for ADHD, National Resource Center for ADHD
 Report 10 of the Council on Science and Public Health: Attention Deficit Hyperactivity Disorder, 2008, June 2, CSAPH, American Medical Association
 "What Causes ADHD?", Last reviewed 2008, June 26, National Institute of Mental Health
 "International Consensus Statement on ADHD, 2002, Jan, Clinical Child and Family Psychology Review
"Adult ADHD Underdiagnosed", 2008, May 6, Charlene Laino, MedicineNet.com
 "ADHD Underdiagnosed in Girls", 2000, April 1, Ellen B. Littman, Family Practice News