The findings suggest patients aren't told enough about how to take care of themselves and stay healthy before they go home, the researchers said. A few simple things - like making a doctor's appointment for departing patients - can help, they said.
The study found that a surprising half of the non-surgery patients who returned within a month hadn't even seen a doctor between hospital stays.
"Hospitals put more effort into the admission process than they do into the discharge process," said Dr. Eric Coleman, one of the study's authors from the University of Colorado in Denver.
Coleman, who runs a program to improve "hand-offs" between health care systems, said patients often have a honeymoon notion about how things will be once they're home. Then when they become confused about how to take their medicine or run into other problems, they head back to the hospital because they don't know where to turn, he said.
The issue of hospital readmissions and their cost has come under scrutiny in recent years. And it's getting attention now because President Barack Obama's budget calls for reducing spending on Medicare readmissions to pay for health care reform.
For their study, reported in Thursday's New England Journal of Medicine, the researchers looked at Medicare records from late 2003 through 2004. They found that about 20 percent of 11.9 million patients were readmitted to the hospital within a month of discharge; about a third were back in the hospital within three months.
About half of the patients hospitalized for ailments didn't see a doctor before they landed back in the hospital within a month.
Patients with heart failure and pneumonia had the most readmissions overall; among surgical procedures, heart stents and major hip and knee surgery had the highest returns.
About 10 percent of all readmissions were probably planned, such as putting in a stent, the researchers said. They estimated that the cost of unplanned return visits in 2004 was $17.4 billion.
"It's a big hunk of money and it's a big hunk of misery," said another study author, Dr. Stephen Jencks, an independent consultant who worked for the Centers for Medicare and Medicaid Services.
Besides making follow-up doctor appointments, Jencks said hospitals should give patients a list of all their medications, explain what to do at home and where to call if they run into problems. He said the hospitals should also call the patient within two days and make sure that the patient's doctor knows they were in the hospital.
He said the goal is to keep patients from getting really sick again, not to keep them out of the hospital if they do.
The differences in readmission rates among states suggests that improvements can be made, he said. Iowa had the lowest rate with 13 percent, while Washington, D.C., had the highest at 23 percent.
Dr. Brian Jack at Boston Medical Center tells the story of a patient who didn't understand that the blood pressure medicine that the hospital told her to take was the same as the one she had at home - just with different names. She took both and returned to the hospital with kidney failure. Jack and his colleagues tested a new checklist that nurses used when they sent patients home. The patients who used the checklist had 30 percent fewer visits to the emergency room or return hospital stays over the next month, compared to patients who didn't use it, they found.
"There are not too many things that improve health and save money," said Jack, who was not involved in the new research.
In 2007, a panel that advises Congress on Medicare suggested ways to cut hospital readmissions. One recommendation was to change how Medicare pays hospitals and to cut payments to those with high rates - an approach included in Mr. Obama's budget proposal.
Currently, hospitals get the same payment for each hospital stay and critics say there's no incentive to reduce readmissions.