Dutch researchers, who published their findings in the European Respiratory Journal, found that of 140 patients who had their pneumonia diagnosed by x-ray, doctors initially thought only 41 of them had the severe lung infection.

"That's worse than flipping a coin," said Richard Watkins, who was not involved in the study but has researched how doctors diagnose pneumonia. "I think that's an argument for doing chest x-rays."

People with pneumonia may have a cough, fever, nausea, vomiting, chills or chest pain. Under some circumstances, the infection can put patients into an intensive care unit and even turn fatal.

In 2009, the U.S. Centers for Disease Control and Prevention said that about 1.1 million Americans were hospitalized with pneumonia, and about 50,000 died of it.

According to researchers led by Saskia van Vugt from the University Medical Center in Utrecht, most doctors use their best judgment in deciding who has pneumonia, because it's not possible to give everyone an x-ray to check for signs of the infection.

Little was known, however, about how accurate doctors were with their diagnoses, the researchers wrote.

For the study, van Vugt and her colleagues used information collected between October 2007 and April 2010 on 2,810 adult patients of doctors in 12 European countries.

All of the patients came to the doctor with a cough, but only 72 were initially diagnosed with pneumonia. All of the patients were then given a chest x-ray to see how accurate the doctors' diagnoses were.

Of those 72 initial diagnoses, the x-rays showed that 31 did not have pneumonia. In the rest of the group, the researchers found the doctors missed 99 cases.

Overall, the doctors correctly diagnosed fewer than a third of pneumonia cases.

While catching only 29 percent of pneumonia cases seems alarming, Watkins said there may be differences between how doctors Europe and in the United States diagnose the illness. It's common, for example, to have U.S. doctors order x-rays if they suspect pneumonia, he said.