PRESIDENT Clinton's summit on youth violence was held last week with much attention paid to the possible causes of the recent school tragedies -- guns and media violence making it to the top of the list.

Though these are important issues that intensify the risk for violence, I am troubled by a startling omission from the list of potential causes: the under-diagnosis and under-treatment of mental illness in children and adolescents.

This under-reported factor was brought to light in a report by Dr. Jeffrey Fagan, director of the Center for Violence Prevention Research at Columbia University, in a recent New York Times article. He listed firearms possession, festering grievances and the presence of undiagnosed mental illness as the common denominators of the young perpetrators of the school shootings that have shocked the nation during the past 18 months.

The recent tragedy in Littleton, Colo., underscored my concern about undiagnosed mental illness in our nation's youth. I was particularly disturbed after reading news accounts about the writings of Eric Harris, one of the shooters. As a child psychiatrist, I am trained to spot warning signs, but Harris' writings on the Internet were so confused and troubled, they should have alarmed even those without special training. Even a layman should have concluded that Harris warranted a thorough mental-health evaluation. He was the kind of disturbed youth most vulnerable to copying the violence so prevalent in today's culture.

Who's responsible when disturbed youngsters do not get the psychiatric help they need?

The police and school employees, even when aware of a problem, cannot be expected to thoroughly evaluate teen-agers for mental illness. The school's primary job is to educate, and the police investigate crimes. They cannot be expected to identify every troubled teen-ager.

Evaluations should be done by mental-health professionals skilled in interviewing children and youths. Health professionals are sanctioned to ask a young person and the parents if there is a family history of mental illness, among other confidential questions. The answers to these questions provide information critical to an accurate diagnosis. This kind of information gathering is inappropriate for school and law-enforcement personnel. They are not trained to interpret it or required to hold it in confidence.

As the public debates strategies to prevent school violence, we should discuss the under-diagnosis and under-treatment of child and adolescent mental illness.

Under-diagnosis and under-treatment happen for several reasons:

* First, the stigma against mental illness prevails. Despite considerable effort by parent advocacy groups such as the National Alliance for the Mentally Ill and the Federation for Families, one seldom hears children or adults describe their medication for depression the same way they talk about their asthma or allergy medication. They are often ashamed of needing help. This leaves the parents alone without the support of other parents who are similarly struggling. The children are also left alone to struggle without the peer support they want and need. In some cases, we have learned that they ease their isolation through the Internet, gravitating in cyberspace toward influences that further their decline rather than aid recovery.

* Second, to contain costs, our health care system has restricted access to help. When parents need help for a child, they must see either a pediatrician or a family practitioner before being referred to a mental-health professional. In this era of managed care, large numbers of patients are seen in short visits to pediatricians and family-practice physicians. Ten minutes is the standard time most HMOs allot for a pediatrician to see a child. There is considerable risk that the doctor might not really know the child and will base recommendations only on what he or she sees and hears in the brief office visit.

If the child is referred to a mental-health professional, time pressures again become an issue.

A comprehensive evaluation means gathering a lot of information from several sources and crafting a complete set of recommendations for treatment. A mental illness cannot be cultured and viewed under a microscope.

A diagnosis takes time -- time to talk to the child, to talk to the parents, to call the school, and to get records from other sources such as the police, if charges have been filed.

This is not a one-hour outpatient evaluation but rather a time-intensive, carefully done examination that might require several visits. It might need to occur while the child is hospitalized, to ensure safety and to get the most information possible.

Additionally, parents might be advised to search their children's rooms, where the artifacts of their independent activities and their private lives are stored. This is not an invasion of privacy when there is cause for concern; it is pro-active, concerned parenting, and it is sometimes the only way to learn the depth of a problem.

As we rush to determine ways of preventing school tragedies, the issues surrounding the diagnosis and treatment of children and adolescents with mental illnesses must become part of the public debate. We cannot continue to risk the lives of healthy children by under-diagnosing and under-treating those who need help. Mental illnesses are in many cases successfully treated. Intervention is most effective when the illness is identified early in life.

While some might argue that treating mental illness can be costly, and that there are no guarantees we will always succeed, it is imperative, for the safety and well- being of all children, that we try.

As we rush to determine ways of preventing school tragedies, the issues surrounding the diagnosis and treatment of children and adolescents must become part of the public debate.

Dr. Susan Villani is a child psychiatrist with the Sheppard Pratt Health System.