TUESDAY, Aug. 16 (HealthDay News) -- There was a dramatic increase in the number of children's flu-related hospital stays in the United States between 2000 and 2009, a federal agency says.

Over that time, flu rose from 65th to 10th in the ranking of reasons why children aged 17 and younger go to the hospital, according to a report from the Agency for Healthcare Research and Quality (AHRQ).

The agency also found that skin infections rose from 13th to 7th in the ranking.

In 2009, pneumonia, asthma, acute bronchitis and mood disorders (depression and bipolar disorder) were the leading conditions that led to hospital stays for those aged 17 and younger.

Children accounted for one out of every six hospital stays in 2009 and 9 percent ($33.6 billion) of total hospital costs that year. About 72 percent of children who required hospitalization in 2009 were infants younger than 1, the AHRQ said.

Compared with other hospitalizations in 2009, a child's average hospital stay was shorter (3.8 days vs. 4.6 days) and less expensive ($5,200 vs. $9,200).

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The Nemours Foundation explains to children what happens when they go to the hospital.