How many Americans are both mentally ill and homeless?

There appears to be no clear answer to this question. But some statistics, as well as estimates from experts on the subject, provide some perspective on it.

For example, the U.S. Conference of Mayors Hunger and Homelessness Survey, published in December 2008, included information on homelessness and mental illness in 23 cities between October 2007 and September 2008. On an average night during this time span, and in the 23 cities combined, some 28,000 people were sleeping in emergency shelters or on the streets, and about a quarter of them had a serious mental illness.

According to the National Coalition for the Homeless fact sheet published in June 2008, 16 percent of the single adult homeless population has a severe and persistent mental illness.

"Depending on where you read, between a third and two-thirds of homeless people are mentally ill and/or use drugs or alcohol," Van Yu, M.D., who heads up the Project for Psychiatric Outreach to the Homeless in New York City, told Psychiatric News. "And a significant percentage have both. So a conservative estimate is that 15,000 of the 38,000 homeless people in New York City are mentally ill and/or use drugs or alcohol."

"Most estimates put the number of homeless people in the United States around 3.5 million a year," Sam Tsemberis, Ph.D., founder of Pathways to Housing for the homeless mentally ill population, said in an interview. "The question then is, How many of these homeless 3.5 million people are severely mentally ill? Depending on how you define mental illness—the criteria you are using—the estimates range between 20 percent and 40 percent. So between 20 percent and 40 percent of the estimated 3.5 million homeless are mentally ill. In other words, anywhere between 700,000 and 1.4 million Americans are both mentally ill and without a home."