The increase is due to supply and demand, said Dr. Stephen Pitts, the lead author of the report by the Centers for Disease Control and Prevention.
"There are more people arriving at the ERs. And there are fewer ERs," said Pitts, an associate professor of emergency medicine at Atlanta's Emory University.
Overall, about 119 million visits were made to U.S. emergency rooms in 2006, up from 90 million in 1996 - a 32 percent increase.
Meanwhile, the number of hospital emergency departments dropped to fewer than 4,600, from nearly 4,900, according to American Hospital Association statistics.
Another reason for crowding is patients who are admitted to the hospital end up waiting in the ER because of the limited number of hospital beds, Pitts added.
A shortage of surgical specialists also contributes. So, too, does the difficulty many patients have in getting appointments to doctor's offices - which causes some to turn to emergency departments, experts said.
"It takes me a month to get an appointment for my own doctor, and I'm a physician, for God's sake," said Dr. Ricardo Martinez, an Atlanta trauma physician. He is executive vice president of Schumacher Group, an organization that manages about 140 hospital emergency departments.
The amount of time a patient waited before seeing a physician in an ER has been rising steadily, from 38 minutes in 1997, to 47 minutes in 2004, to 56 minutes in 2006.
Pitts added that 56 minutes may be the average, but it's not typical: The average was skewed to nearly an hour because of some very long waits.
"Half of people had waiting times of 31 minutes or less," Pitts noted.
Researchers also found that there has not been any recent increases in the number of patients arriving by ambulance, or in the number of cases considered to be true emergencies.
Black patients visited emergency departments at twice the rate as whites in 2006. Among age groups, the highest visitation rates were for infants and elderly people aged 75 and older.
About 40 percent of ER patients had private insurance, about 25 percent were covered by state programs for children and about 17 percent were covered by Medicare, the report found. About 17 percent were uninsured.
Some more findings: Summer and winter were the busiest season in ERs, and the early evening - around 7 p.m. - tended to be the busiest time of day. There were geographic differences as well, with hospitals in the South having the highest ER visitation rates.
Also, half of hospital admissions in 2006 came through emergency departments, up from 36 percent in 1996.
"The ER has become the front door to the hospital," said Pitts, a fellow at the CDC's National Center for Health Statistics.
Some doctors said the report supports a call for increased governmental funding for hospital emergency services.
"Millions more people each year are seeking emergency care, but emergency departments are continuing to close, often because so much care goes uncompensated," Dr. Linda Lawrence, president of the American College of Emergency Physicians, said in a statement.
"This report is very troubling, because it shows that care is being delayed for everyone, including people in pain and with heart attacks," her statement added.
The results are based on a national survey of 362 hospital emergency departments.