Chronic fatigue syndrome has long been the Rodney Dangerfield of illnesses: from many in the medical establishment, it got little -- or no -- respect.

But recently some influential physicians who had been skeptical about the existence of the immunological illness -- derisively nicknamed the "yuppie flu" -- have reconsidered. Last year the Centers for Disease Control, which investigates outbreaks of mysterious illnesses and epidemics, launched a study of the controversial syndrome. Scientists hope to come closer to discovering a cause or developing a diagnostic test for it.

The first 100 cases of chronic fatigue syndrome were reported in the mid-1980s in Lake Tahoe, Calif. Those sufferers, and many others since then, were high-achieving professionals in their thirties whose debilitating symptoms resemble a severe and unusually persistent flu: headaches, joint pain, memory loss, periods of disorientation, swollen lymph nodes and, above all, crushing fatigue.

A CDC team headed by Walter J. Gunn is focusing on four cities -- Reno, Wichita, Atlanta and Grand Rapids -- and also are examining 370 patients referred by physicians. Among them is a marathon runner who barely had the strength to walk to her car.

Because the illness mimics the symptoms of other diseases, such as the lassitude that often accompanies depression, many doctors were skeptical that chronic fatigue was a distinct illness.

Doctors soon pointed to the fact that unlike depression, which tends to occur gradually, chronic fatigue syndrome had a sudden, clear beginning.

In the past year, the CDC team and other researchers who have studied the illness have gathered evidence suggesting that chronic fatigue is not just a physical manifestation of an underlying psychological problem.

Jay A. Levy, a prominent AIDS researcher at the University of California at San Francisco and his colleagues have found that patients with the syndrome have overactive immune systems. A University of Miami researcher has reported that the immune systemps natural killer cells, which help the body fight infection, are weakened in some people who appear to be suffering from chronic fatigue.

A group led by Anthony L. Komaroff at Boston's Brigham and Women's Hospital said it has found a herpes virus in 70 percent of a sample of chronic fatigue patients but in only 10 percent of normal people.

And several months ago, a team from the Wistar Institute in Philadelphia announced what may be a link between a previously unknown retrovirus, a group of viruses that includes AIDS, and chronic fatigue syndrome.