Nearly half of all Americans with a severe mental illness do not seek treatment, although most would benefit significantly if they did, the surgeon general will report today.

The report points to social stigmas about mental illness and to financial and other barriers to treatment as the main reasons why so many people are not being helped. The best way to reduce stigma, the report said, is to educate people about how successful treatment can be.

"Effective interventions help people to understand that mental disorders are not character flaws but are legitimate illnesses that respond to specific treatments," the report says in its summary.

While Surgeon General David Satcher makes no direct recommendations--except that people with mental disorders should seek help--he does support calls for full "parity" in mental health coverage, saying it is "an affordable and effective objective." Over the summer, President Clinton required all federal employee health plans to offer the same coverage for mental disorders as for other illnesses, but many other private plans still offer considerably less mental health protection.

About 20 percent of Americans are believed to experience a mental disorder each year, but about 5 percent are severe illnesses such as manic depression, schizophrenia and obsessive-compulsive disorder.

The report is based on a review of more than 3,000 research papers, and on information gathered at the White House Conference on Mental Health in June, chaired by Tipper Gore.