Harriet, 45, grappled with many midlife problems. But her foremost concern, she says, was feeling neglected by her husband. "I needed affirmation of my worth, validation of my femininity."

To get rid of extra pounds and tension, the Springfield psychiatric social worker started running, sometimes alone, but often with whomever happened to be at the local high school track. During a series of runs with a married man, Harriet became intrigued by his jocular -- but persistent -- suggestions that he could add excitement to her life.

Four months later Harriet said, "I'm ready." They saw each other whenever possible for six months -- when her lover abruptly terminated their relationship. "I was devastated," she remembers. "I kept asking myself why I had gotten involved with a casual acquaintance."

Looking back, Harriet says her problems were far deeper than surface boredom. "I was starving in my marriage. I just wasn't special at home.

"I now think people have an affair because they have allowed some-thing at home to die or to go dormant. Then they try to fill that vacuum or empty space with someone else."

The reasons, say experts, for middle-age infidelity -- after two decades of marriage -- differ for the sexes.

"For many women," says Dr. Jon Meyer, psychiatry professor at the Medical College of Wisconsin and former director of the Sexual Behaviors Consultation Unit at Johns Hopkins Hospital in Baltimore, "their husbands simply aren't paying as much attention to them as they used to."

Estelle, 48, a Silver Spring office manager, says she became involved with an old and good friend because "I had an emotional void." Her husband, she says, never expressed any romantic feelings toward her. "Very few men woo the woman. I think the relationship before the sexual part and after the sexual part is more important than the time in bed."

Estelle maintains her affair helped her self-esteem. "It made me feel like a woman."

Tampa psychologist E. Michael Lillibridge, marriage and family therapy specialist and author of The Love Book for Couples (Humanics Ltd., Atlanta, 1984, $12.95), hears such stories often. "When a woman seeks an affair," he says, "she is seeking personal, emotional acceptance. She wants to be appreciated, valued as a person, not as a sex object."

Women who come to him for counseling during an affair, he says, explain, " 'Well, we have sex, but that's not the meaningful part for me. It is secondary to the fact that I feel this individual cares about me and wants to spend time with me.' "

While women have affairs to be appreciated, says Lillibridge, "men have them to feel valued sexually. The man wants to be flirted with, cared about and perceived in sexual masculine terms. They want the woman to like them for their good looks, to see them as sexually satisfying."

And, concerned about growing older, men want to "prove that they can be attractive to women," says Meyer, "usually younger women." Do affairs ever improve a marriage? "I've seen some marriages enhanced by affairs," says licensed clinical social worker Anita H. King of Bethesda. "That is not the general rule."

Often, the "empty-nest syndrome" gives both sexes time to reflect on the marriage and unresolved differences. "The basic weaknesses in the relationship come into focus," says King.

"Women who never got their emotional needs met by their husband turn to their children," she says. "The husband is off working, living in his own world. While he brings home his paycheck, eats and sleeps there, the woman's dynamic, intense emotional interactions are with the children."

When the children leave, maintains Washington, D.C., family therapist Marion L. Usher, it is time for the couple to "forge ahead together, develop new bonds, new ways of relating, different schedules, making things meaningful for them." Instead, some men and women find it easier to seek companionship outside of the marriage.

A midlife affair, say experts, differs from the type younger people have. Preoccupied with raising children, establishing careers, gaining recognition and becoming successful, people in their twenties and thirties have more casual affairs, "more on the side," says Lillibridge. But "when we reach midlife, we turn inward, try to understand ourselves more. Then an affair takes on much more prominence."

"There is a greater temptation in midlife to have affairs," says Meyer. After many years together, some people become "jaded or used to their spouse, experiencing very little excitement or novelty in the sexual life."

"It is as though all the original heat, fire, love, passion, devotion and commitment have slowly either been simmered out or been worn down," says King. "Some men just go looking for that feeling of excitement -- the intensely pleasurable feelings of new passion."

"I've seen some situations where the relationship was basically good and committed and loving, but due to military service, job assignments out of town, sickness or some kind of medication that got in the way, one of the spouses wasn't available sexually to the other. The affair helped get the couple through that period.

"The bottom-line problem with affairs usually involves some kind of dishonesty," King says. "And whether that dishonesty affects the individual, her spouse or family, I'm committed to the notion that life works better for everybody if we can be honest."

"Affairs," says Usher, "are always a signal that something is not going well either for the person or in the relationship. If you want to make a comparison, it would be that fever tells you something is wrong in the body. Affairs are an indication of an emotional aspect of a person that is out of kilter."

People who become involved in an extramarital liaison may be bored, depressed or experiencing an emptiness in their marriage. But since "affairs are stimulating, heighten people, make them feel better," Usher observes that the individual rarely seeks professional help while involved in an illicit triangle.

When the affair ends -- usually in less than a year -- those involved are "depressed, troubled and worried," she says. They walk into her office feeling depleted, angry, betrayed. "When they come to me, they want to decide whether they are going to rebuild and repair the marriage or separate and divorce."

Because "too much energy is taken away from the therapy," Usher would not knowingly work with a couple if one partner is straddling two relationships. "They would be wasting their money because the affair is stronger than the idea of working out the problems right now."

The problems are rarely just sexual. One of the conflicts Harriet and her husband contend with in private and in group counseling stems from his demanding job. While she silently tolerated his physically and emotionally draining schedule for 20 years, she felt "a long-term undercurrent of resentment, both of his work ethic and his striving for success. Workaholic men take the relationship at home for granted. They always feel like they are on the line at work, but the woman will be there for them at home."

Harriet regrets not dealing with problems as they occurred. "We poured our gripes into a gunny sack and held onto them. Carrying those gunny sacks around gives you a good excuse to be angry."

Anger enabled her to rationalize having an affair without remorse or guilt afterward. But she now counsels others: "There is no way you can be involved with two people at the same time."

Estelle recently read a magazine article that claimed an affair enhances a marriage. "I don't see how," she says now. "While it is very easy to have an affair, it's very hard to work on your relationship" with your spouse.

"An affair is a metaphor for marital disharmony," says Usher. "What problems in the marriage is this couple not dealing with, not addressing, not working through, not creating other solutions for? Each situation is unique."

"If you're contemplating an affair," advises Lillibridge, "I would recommend that you first consider what you could do differently in your marriage. Sit down, start to talk, really communicate.

"Our ultimate goal," he says, "is to have truly intimate relationships with other human beings. We all -- men and women -- have a lifelong desire for closeness, sexual and emotional intimacy."

Says Harriet: "An affair is painful, difficult. It is also not dealing with the real issues. The problems you had in your marriage come with you to the affair. The marriage plunges into a downward spiral. What starts out as the solution to the problem becomes a problem."