Most of these kids suffer from depression, bipolar disorder or psychosis. Unfortunately, there are too few trained psychiatrists, psychologists or hospital beds to treat these children effectively, experts say.
"This is a common and costly problem," said lead researcher Dr. Naomi Bardach, an assistant professor of pediatrics at the University of California, San Francisco.
More than 14 million children and teens in the United States have been diagnosed with a mental health disorder, she noted.
The costs for the most common diagnosis, depression, total about $1.3 billion a year, she added: "That's similar to the hospital costs just for asthma."
The problem has been growing. Between 1997 and 2010, hospitalizations of children for mental problems jumped 80 percent, she explained.
Bardach noted that as children get older, the odds of them developing a mental illness that requires them to be hospitalized increase. "Older kids are much more likely to get admitted for a mental health reason," she said.
The scope of the problem has caused concern in the medical community. "Because of that, there is a big emphasis under the Affordable Care Act as well as under specific legislation to try to better understand how we can serve those kids," Bardach said.
The next step, she said, is to figure out the best treatment for the most common and costly conditions -- depression, bipolar disorder and psychosis. "Then we can try and make all hospitals make sure they're delivering the best care to kids," Bardach explained.
Taking care of these kids once they have left the hospital, and preventing them from being hospitalized in the first place, is as important as how effectively they are treated in the hospital, Bardach added.
"We need to make sure in the outpatient setting they get really good care as well," she said.
Bardach noted that there is a shortage of trained pediatric mental health specialists.
"Mental health issues are more common than people think. It's not something people talk about very much. But the fact is, it's common," Bardach said.
The report was published online March 17 and in the April print issue of Pediatrics.
Rose Alvarez-Salvat, a pediatric psychologist at Miami Children's Hospital, said, "This is a major problem. We are seeing it on so many different levels."
This study looks at only one facet of the problem, she said, since it doesn't take into account the number of children in outpatient care or those seen in schools or community mental health centers.
"This study is an under-representation of the amount of mental health problems that exist with children. Moreover, there aren't enough psychologists for the type of kids being admitted to hospitals," Alvarez-Salvat said.
For the study, Bardach's team used data from the Kids' Inpatient Database and Pediatric Health Information System to look at all hospital discharges in 2009 for patients aged 3 to 20.
Their goal was to determine how often these hospital stays were for mental conditions, how many were from general hospitals and how many were from children's hospitals.
The investigators found that general hospitals admitted more than three times the number of children suffering from mental conditions than children's hospitals.
The most common diagnosis was depression, which accounted for 44 percent of all children admitted for a mental health problem, with associated costs of $1.3 billion, the researchers reported.
Bipolar disorder was the second most common diagnosis, accounting for 18 percent of admissions and costing $702 million, followed by psychosis, at 12 percent and costing $540 million.
Visit the U.S. National Library of Medicine for more on children's mental health issues.