The death of a child not only alters a family forever but also sharply increases the risk that parents will later be hospitalized for a mental illness, researchers are reporting in the largest study to date of parent bereavement and mental health.

The risk is greatest during the first year after the child's death but remains elevated even five years afterward, the researchers found, and includes higher rates of schizophrenia, depression and abuse of drugs and alcohol.

The overall rate of psychiatric hospitalization among bereaved parents in the study was less than 3 percent over five years, but, experts noted, doctors do not usually admit patients for mental illness unless their condition is urgent.

The paper appears in the current issue of The New England Journal of Medicine.

''This is a very important study, because bereavement traditionally has been way underrecognized and undertreated in medicine and psychiatry,'' said Dr. M. Katherine Shear, director of the Bereavement and Grief Program at the University of Pittsburgh School of Medicine, who was not involved in the research.

Because a vast majority of people who grieve the death of a child do so without lasting mental health problems, psychiatrists have not had a good sense of how much, or even whether, the deaths can increase the likelihood of psychological illness, she said, adding, ''This study is large and rigorous enough'' to put those questions to rest.

In the study, Dr. Jiong Li of the Danish Epidemiology Science Center led a team of doctors who analyzed the medical records of more than a million people who had at least one child born before 1999. The records are part of Danish databases that include detailed information on people who have sought treatment for mental health problems. A total of 17,033 parents in the database had lost a child under 18, and 495 of them were hospitalized for psychiatric problems for the first time in the five years afterward.

In the year after the death, bereaved mothers were almost seven times as likely as their nongrieving peers to be hospitalized for a mood disorder, like depression; about four times as likely to be admitted for schizophrenia; and nearly three times as likely to be admitted for abusing drugs and alcohol.

Bereaved fathers were at lower risk than bereaved mothers during this first year, but were still nearly six times as likely as nongrieving fathers to be admitted for a mood disorder like depression, and more than twice as likely for schizophrenia.

With time, the difference in risk between the bereaved parents and their peers narrowed significantly, but after five years the bereaved group still had a higher rate of hospitalization for psychiatric problems -- about 40 percent higher in fathers, nearly 80 percent in mothers.

Neither the child's age nor the parents' age at the time of death significantly affected the risk of hospitalization, but parents who had lost an only child were at higher risk than those who had larger families.