What percentage of the U.S. jail and prison population is mentally ill?

Answer: One in four or one in two, depending how you count.

A follow-up to the post below. This is from the excellent Frontline investigation a few weeks ago called "THE NEW ASYLUMS":

Of the nearly 2 million inmates being held in prisons and jails across the country, experts believe nearly 500,000 are mentally ill. According to the National Alliance for the Mentally Ill (NAMI), 16 percent of the prison population can be classified as severely mentally ill, meaning that they fit the psychiatric classification for illnesses such as schizophrenia, major depression, and bipolar disorder. According to staff at city and community jails, 25 percent of the jail population is severely mentally ill. However, when other mental illnesses, such as anti-social personality disorder, borderline personality disorder and depression, are included, the numbers are much higher, and NAMI puts the number of inmates suffering from both mental illness and substance abuse the percentage at well over 50 percent.