"I was trying to get anorexia," the teen-age girl confided to a therapist. "But I didn't. So instead I got bulimia."

The youngster was very complacent about the eating disorder she had "gotten," Dr. Sue Bailey said recently, suggesting that despite their potential for sudden death, "even having these diseases is a fad. They are the in thing to have."

Bailey heads the Eating Disorders Clinic at the Washington Hospital Center and, in a survey she made recently of teen-agers -- 13- to 19-year-olds -- at three private schools in the Washington area, she discovered a widespread attitude that the disorders were no more than another way to diet. As many as 45 percent of college freshmen said they would consider trying bulimia to lose weight, and on that college campus she discovered the existence of so-called "pig-out parties" at which about 10 young women were engaging regularly in group gorge-and-purge routines as an acceptable, even attractive, method of weight maintenance.

Anorexia is a condition of self-starving and obsessive exercise combined with a body image perception so distorted that the merest curve is regarded as repugnant fat to be eliminated at all cost. "All cost" is sometimes the life of the anorexic.

Anorexics are skeletal in appearance, often looking younger than they are, losing the normal early signs of femininity that they perceive as fatness. Many either stop menstruating or fail to start. Some damage their reproductive systems.

Bulimia is the obsessive, repeated cycle of binging enormous amounts of fattening foods -- cakes, candy, breads, french fries, cookies, butter by the stick, eventually anything that can be stuffed into the mouth, sometimes 20,000 or more calories in one sitting -- and then purging it either by self-induced vomiting, laxatives or diuretics.

Bulimia is also accompanied by distorted body image and a phobic attitude toward becoming fat. Bulimics may or may not appear anorexic in appearance. The conditions may coexist, as, it is believed, they did in the late singer Karen Carpenter, who died in 1983. But even when bulimics are at normal weight, the disorder is potentially lethal. Both bulimia and anorexia involve addictive behaviors; once entrenched, they are as difficult to withdraw from as any drug.

A recent Gallup poll, sponsored by the Comprehensive Care Corp., which operates a chain of treatment centers, confirmed what specialists had been speculating for some years -- that both disorders are widespread among teen-age girls and young women, perhaps 1 million teen-agers and twice that many women under 40. More females are affected than males.

The poll also suggests that these genuine disorders are achieving social acceptance despite their well-publicized health hazards.

"It is," says Sue Bailey, "a kind of Russian roulette."

Dr. David Jimerson, a biological psychiatrist who is chief of the eating disorders program of the National Institute of Mental Health, believes that more than society's preoccupation with thinness may be involved.

Work at NIMH on the biology of depression has uncovered a possible link to eating disorders. For example, says Jimerson, "bulimic and anorexic patients have a high frequency of major depressions, with estimates that up to 70 percent of eating disorder patients have a lifetime prevalence of major depression."

"They also have an increased frequency of alcoholism and other drug abuse," says Jimerson, and, some studies suggest, "possibly an increased frequency of anxiety disorders."

In a presentation to colleagues last month, Jimerson also noted that eating disorder patients "have an increased family history of depression, which is one line of evidence which suggests a biological predisposition to eating disorders."

Work at NIMH on depression as well as eating disorders points toward a possible dysfunction of the system involving the brain chemical serotonin, a neurotransmitter, or brain messenger, that has been linked to a variety of states, both of body and mind. It is involved with pain control, but also with mood and the feeling of satiety after eating.

There are also studies that link decreased serotonin functions to impulsive behaviors, including suicide attempts, another characteristic of many with eating disorders.

And some bulimic patients respond well to a group of antidepressive drugs known as tricyclics. These drugs are known to enhance the serotonin system and have been useful in some chronic pain and headache patients as well as in depressives.

All of these point to the possibility of a serotonin connection in eating disorders, but Jimerson emphasizes that they suggest research directions rather than conclusions.

There is considerable discussion in the psychiatric community not only over treatment for the disorders, but also over their diagnosis. A proposal under consideration would permit a formal diagnosis of both anorexia and bulimia at the same time in the same patient (now a patient can have only one or the other), and would establish a specific number of binge-purge cycles -- say, for example, three a week -- to permit a diagnosis of bulimia.

Bulimics have been treated with a variety of drug, behavioral and psychotherapeutic techniques, and although, says Jimerson, "there is data showing that antidepressant drugs are effective in some subgroups, there is also data from psychotherapy and behaviorial therapy studies that those treatments may be effective as well. I think an important issue right now is that we don't know which approach is better for which subgroup.

"I think most clinicians would tend to agree that pharmacological treatments at least initially, should be reserved for patients with symptoms that are more severe" or those that do not respond to other approaches.

Jimerson does not dismiss the role of "socio-cultural issues around food, dieting, fasting and how that interfaces with a possible biological vulnerability in some people."

But, he suggests, if there hadn't been the emphasis on thinness, the biological vulnerability might have resulted in the individual's focusing on something else -- alcohol, drugs, whatever any particular culture might suggest.