Residents of cities in the Northeast and California have at least three to six times the risk of being diagnosed or hospitalized with schizophrenia and other serious mental disorders, according to a new study by researchers at Washington's St. Elizabeths Hospital.

Scientists have long debated whether mental illnesses are evenly distributed throughout the population or cluster in geographic areas or among specific groups.

The new review of nine separate studies, conducted from 1880 to 1963 and involving nearly 750,000 patients, suggests that schizophrenia and other forms of serious mental illness may occur more often in cities that have the greatest population density.

Those who may especially be at risk are poor people who live in urban areas in the Northeast and in California, said E. Fuller Torrey, lead author of the study. The report appears in last December's issue of Schizophrenia Bulletin.

"It looks like some kind of crowding factor has to do with the transmission of the disease," said Torrey, a psychiatrist at the National Institute of Mental Health's Neurosciences Unit at St. Elizabeths Hospital.

Scientists do not know what causes serious mental illnesses but suspect they result from a combination of genetic and environmental factors.

As early as 1903, psychiatrist William A. White reported that a disproportionate amount of mental illness was found in cities. But some researchers suggested that the higher rate of disease could be explained because people move to the cities seeking care and because behavior considered unconventional is more often tolerated in urban areas.

But Torrey said that based on his review of the studies, the pattern "cannot all be explained by migration to urban areas, as people have thought."

Those suffering from schizophrenia in urban areas tend to come from the lowest socioeconomic groups, he said.

It appears that overcrowding and stress help contribute to the higher proportion of the disease in cities. That is consistent with a study that found a high level of schizophrenia in western Ireland prior to World War II, a rural area composed of large, impoverished families who lived in overcrowded conditions.

While socieoeconomic factors alone are not believed to cause schizophrenia or other mental disorders, they could play a role in triggering them, Torrey said.