Although the term "cyberchondria" emerged in 2000 to refer to the practice of leaping to dire conclusions while researching health matters online, a recent Microsoft study is the first systematic look at the anxieties of people doing searches related to health care, according to Eric Horvitz, an artificial intelligence researcher at Microsoft Research.

If that headache plaguing you this morning led you first to a Web search and then to the conclusion that you must have a brain tumor, you may instead be suffering from cyberchondria.

On Monday, Microsoft researchers published the results of a study of health-related Web searches on the company's Live search engine as well as a survey of the company's employees.

The study suggests that self-diagnosis by search engine frequently leads Web searchers to conclude the worst about what ails them.

The researchers said they had undertaken the study as part of an effort to add features to Microsoft's search service that could make it more of an adviser and less of a blind information retrieval tool.

Although the term "cyberchondria" emerged in 2000 to refer to the practice of leaping to dire conclusions while researching health matters online, the Microsoft study is the first systematic look at the anxieties of people doing searches related to health care, Eric Horvitz said.

Horvitz, an artificial intelligence researcher at Microsoft Research, said many people treated search engines as if they could answer questions like a human expert. Horvitz is also a computer scientist and has a medical degree, and his fellow investigator, Ryen White, is a specialist in data retrieval technology.

They found that Web searches for things like headache and chest pain were just as likely or more likely to lead people to pages describing serious conditions as benign ones, even though the serious illnesses are much more rare.

The researchers said they had not intended their work to send the message that people should ignore symptoms. But their examination of search records indicated that researching particular symptoms often led quickly to anxiousness.

They found that roughly 2 percent of all Web queries were health-related, and about 250,000 users, or about a quarter of the sample, engaged in a least one medical search during the study.

About a third of the subjects "escalated" their follow-up searches to explore serious illnesses, the researchers said.

Of the more than 5,000 Microsoft employees who answered a survey on their medical search habits, more than half said that online medical queries related to a serious illness had interrupted their day-to-day activities at least once.

The researchers said that Web searchers' propensity to jump to awful conclusions was basic human behavior that has been noted by research scientists for decades.

In 1974, the psychologists Amos Tversky and Daniel Kahneman wrote a seminal paper about decisions that are based on beliefs about the likelihood of uncertain events, like the outcome of an election or the future value of the dollar.

They said that people usually employ common sense rules to aid in decisions. The rules can be quite useful, but they also frequently lead to systematic errors in judgment.

The Microsoft researchers noted that reliance on the rankings of Web search results contributes a similar bias to the judgments people make about illness.