A scrawled note filled the movie screen: "Our children know what rape is and how it is done. Our daughter will know what it field (sic) like to be rape naket (sic)."

The daughter, said Gene Abel, a psychiatrist who deals exclusively with sexual abuse cases, was 3 years old and a victim of rape. The rapist -- and the author of the statement -- was her 22-year-old father, who had sought help from Abel.

Abel was one of several mental health professionals who gathered in Reston last week to discuss the treatment of sex offenders and their victims at a conference sponsored by the Northwest Center for Community Mental Health. Organizers said the two-day conference, entitled Sexual Aggressors and Their Victims, was called in response to the increasing number of sexual assault cases being reported throughout the nation.

Of particular concern to conference participants was the increase in reported incidences of rape and incest.

James Brieling, a psychologist at the National Institute of Mental Health, said the number of known rape and incest cases has doubled within the past few years. Last year, Brieling said, the FBI recorded more than 60,000 rapes and more than 6,000 cases of incest. By comparison, in 1977, fewer than 3,000 instances of incest were reported.

Brieling said many more sexual assaults are being reported because of the women's movement's efforts to increase public awareness of the trauma associated with such abuse.

Another factor in the higher number of reported cases, conference speakers said, is a 1972 federal child abuse law, which prohibits sexual abuse of children by family members, and directs states to order protection of children involved in such cases.

Speaker after speaker at the conference emphasized, however, that the statistics represent only the tip of the iceberg. The majority of sexual assault cases, Brieling said, still go unreported.

"Despite the ubiquitousness of sex in this culture," he said, "in most circles it is still considered a taboo to discuss sex, much less make it public that you have been abused."

This same attitude, Brieling said, is reflected in the scarcity of programs designed specifically to deal with sex offenders, although many mental health agencies deal with sexual abuse as part of their overall programs. The latest study conducted by the Law Enforcement Assistance Administration found there were only 20 special programs for dealing with sexual abuse in the nation, including 16 begun within the past five years.

A lack of understanding and a shortage of such programs, Abel said, will exacerbate an already staggering problem. Conservative estimates, Abel said, are that for every rape reported two other women have been assaulted but have not reported it. And, said St. Elizabeths's social worker June Siegel, for every incident of incest reported, 10 others go unreported. According to conservative federal estimates, 100,000 children were sexually abused last year.

"I am convinced," said child psychiatrist Ira Lourie, "that there are areas in this country where there is an incest epidemic."

Lourie, who is assistant director of children's mental health services at the National Institute of Mental Health, said incest cuts across all economic lines. The only common characteristics among families where incest occurs, he said, are isolation from the community and a breakdown in family relations.

Lourie, who also aids the Montgomery County Protective Services, said that office cannot keep up with its caseload. "When they call it a bedroom community," he said wryly, "they really mean it."

One reason many incest cases go unreported, Siegel said, is that the children who are victims are overcome by guilt and embarrassment.

Typically, he said, the child feels if the relationship is revealed, the family will be destroyed, and that it will be the child's fault. The child is confused -- the parent involved is the only mother or father they know -- and usually will not turn to a teacher or an adult confidante until several years after the onset of the relationship.

"A child has been taught," said Lourie, "to say no to strangers, but he has never been told to say no to Mommy or Daddy."

Although more than three quarters of recorded incest cases involve father-daughter relationships, Brieling said studies have found that father-son, mother-son and brother-sister relationships may occur more frequently than previously believed.

Under Virginia law, when incest is reported one of the child-protection agencies will contact the family and begin counseling. If the parent refuses to participate, criminal charges will be filed. Unlike the District, where the child or parent is removed from the home until the problem can be solved, the Virginia family usually remains together.

Abel said with proper treatment, the repeat rate among rapists is reduced to 10 percent. Without it, Abel warned, rapists demonstrate one of the highest recidivism rates of all criminal offenders -- 35 percent.