Preliminary results from the world's largest survey on mental health indicate that mental illness is widespread and undertreated, and that wealthy people with mild illness receive more and better treatment than poor people with severe illness.

From 1 to 5 percent of the populations of most of the countries surveyed had serious mental illness, according to the findings, being published today in The Journal of the American Medical Association.

And in most of the countries, 9 to 17 percent of those interviewed had had some episode of mental illness in the last year, whether serious or less severe, said the study, by researchers from the World Health Organization and Harvard Medical School.

Around the world, the authors found, mental illness causes as many lost days of work as any physical problem like cancer, heart attack or back pain.''The level of role impairment we found to be associated with serious mental disorders was staggering: more than a month in the past year when the respondents reported being totally unable to work,'' said one chief author, Dr. Ronald C. Kessler, a professor of health care policy at Harvard.