Dr. Leslie Hillel Farber, 68, a noted psychoanalyst and a former chairman of the Association of Existantial Psychology and Psychiatry, died Tuesday at St. Luke's Hospital in New York City following a heart attack.

Dr. Farber, who lived and practiced in Washington from 1953 to 1968, was a classically trained psychoanalyst who was profoundly aware of the lasting strength of the work of Sigmund Freud in founding the discipline. Yet he was a great synthesizer of ideas and a leading theoretician of existential psychoanalysis.

What existentialism has to offer to psychoanalysis is the notion that man is what he is, that, for example, his life may be conditioned -- and diminished -- by guilt. For the analyst, this offers a philosophical underpinning for the idea that guilt, perhaps, is not wholly bad or unnatural. It follows that guilt can be modified in constructive ways to reduce suffering.

Dr. Farber, a cheerful and eclectic man of wide learning, conveyed these ideas to generations of students in Washington and in a series of elegant writings in professional journals as well as in The New York Times, The Atlantic Monthly and other publications with general readerships.

Dr. Farber studied psychoanalysis at the old Baltimore-Washington Psychoanalytic Institute under Dr. Harry Stack Sullivan. From Sullivan, he came to believe in what his mentor called the importance of "interpersonal relationships" in the development of human behavior.

By the same token, he was a lifelong admirer of the philosopher Martin Buber and tried to apply the teachings of Buber to his own work. Buber defined human relations in two ways. The first was the "I-It," relationship, which Buber said was characteristic of such hard tasks as making a living and in which other persons or objects are a mere "It." The second is the "I-Thou" relationship, in which people feel affection and mutual respect and acquire a sense of each other's uniqueness.

In 1958, Dr. Farber arranged for Buber to lecture in Washington. The doctor said in an interview at the time that Buber's "I-Thou" formulation "adds a new dimension to the concrete problems of psychiatry." He added that the views of Buber and Sullivan formed a sharp contrast to the more orthodox theory of "an isolated self, abstracted from all present experience and studied in relationship to its childhood past."

As a proponent of existential psychoanalysis, Dr. Farber shared a leading place with Dr. Rollo May. Dr. Farber was chairman of the Association of Existential Psychology and Psychiatry from 1963 to 1977.

Dr. Farber also was known for his theories on human will. He set these out in "The Ways of the Will: Essays Toward a Psychology and Psychopathology of Will," which was published in 1966. Frustration of a conscious will, he observed, can produce anxiety. As in other situations that produce neuroses, he said, a patient can come to understand this process and minimize the cost it imposes.

Dr. Farber was born in Carlsbad, N.M., and grew up in Douglas, Ariz. He received his undergraduate degree at Stanford University in 1934 and his medical degree there in 1938. A year later, he moved to Washington to study with Sullivan at the Baltimore-Washington Psychoanalytic Institute.

In 1942, he joined the U.S. Public Health Service for World War II service. He rose to the rank of lieutenant commander and helped organize the department of psychiatry at the U.S. Marine Hospital at Norfolk, Va. From 1946 until 1953, when he moved to Washington, he practiced in San Francisco.

After moving here, in addition to conducting a private practice, Dr. Farber was a training and supervisory psychoanalyst at the Washington Psychoanalytic Institute until 1968. From 1955 to 1960, he was chairman of the faculty of the Washington School of Psychiatry.

From 1968 to 1969, Dr. Farber was director of therapy at the Austin Riggs Center at Stockbridge, Mass. He then moved to New York City and was in private practice there until his death. He was known among his colleagues for his success with patients who were artists or writers.

In addition to his book on the will, Dr. Farber published "Lying, Despair, Jealousy, Envy, Sex, Suicide, and the Good Life" in 1976.

His marriage to Marjorie Farber ended in divorce.

Survivors include his wife, the former Anne Clague, of New York City; one child by his first marriage, Dr. Stephen P., of Washington; three children by his second marriage, Luke, Seth and Phoebe, all of New York City, and a brother, Manny, of La Jolla, Calif.