TUESDAY, March 12 (HealthDay News) -- The death rate among Danish men in same-sex partnerships has dropped significantly since the 1990s, but the death rate among women in same-sex unions is increasing, a new study finds.

In 1989, Denmark became the first country to allow registered same-sex partnerships. For several years afterward, the death rate among same-sex partners was markedly high, the researchers said.

However, the death rate among men in same-sex partnerships has declined since 1996 and is now below that of unmarried or divorced men. The development of effective treatment for HIV/AIDS is a major reason for the decrease, according to the study, published online March 12 in the International Journal of Epidemiology.

In contrast, the researchers found that the death rate for women in same-sex partnerships has increased, mostly because of suicide and cancer.

"Lesbians may constitute a largely unnoticed high-risk population for suicide and breast cancer, so our findings call for efforts to identify the underlying factors responsible and ensure access to basic health care in this population," study lead author Morten Frisch said in a journal news release.

It has long been known that married people have a reduced rate of death, but significant changes in marriage have occurred in Western nations over the past decades, the researchers noted.

For example, there have been gradual declines in the percentage of people in traditional marriages; increases in the percentages of unmarried and divorced people; a growing number of single people; and fewer people living with a member of the opposite sex.

As in previous research, this study found that being married or living with a member of the opposite sex was associated with lower death rates. From a public health standpoint, it is important to try to identify the reasons why people in these relationships have reduced death rates, Frisch said.