Psychiatric patients accounted for 12% of convictions for unlawful killing in England and Wales between 2003 and 2005, a study has found.
People with mental health problems were also two and a half times more likely to be murder or manslaughter victims than members of the general population, scientists said.
Over the three-year study period, 1,496 people in England and Wales were killed in homicides, the research showed. Of these, 6% had been under the care of mental health services in the year before their death.
A third of the patient victims were killed by other individuals suffering from mental illness.
Study leader Professor Louis Appleby, from the University of Manchester, said: "Historically, society has been more concerned about the risk of patients committing violence than the vulnerability of patients to violent acts. However, our findings show that specialist mental health providers in England and Wales can expect one of their patients to be the victim of homicide roughly every two years."
In almost all 23 cases where a patient was killed by another patient, victim and perpetrator were known to each other as partners, family members or acquaintances .
Victim and perpetrator were undergoing treatment at the same NHS trust in all but two of these cases.
Alcohol, drug use and a history of violence were common among killers and victims.
The findings, published in The Lancet Psychiatry journal, came from the National Confidential Inquiry into Suicide and Homicide by People with Mental Illness, which looked at data on all homicides in England and Wales between January 2003 and December 2005.
Prof Appleby said: "Assessing patients for risk of suicide and violence is common practice, but screening for risk of becoming victims of violence is not.
"Understanding that a patient's risk can depend on the environment they are in - for example their use of alcohol or drugs, or their contact with patients with a history of violence - and properly assessing these risk factors should become a key part of clinical care plans."