And a "surprising" 70% of that group are married – and less likely to use condoms or to have been tested for HIV than those who identify themselves as gay, said Dr. Susan Blank, assistant commissioner at the city Health Department.

Most live outside Manhattan, and many are minorities, foreign-born or of low economic and educational status, according to the survey, conducted in 2003 and published in today’s Annals of Internal Medicine.

"We expected a substantive percentage [to be married], but we hadn’t estimated 70%. No one should be making assumptions. Marital status isn’t necessarily an indicator of sexual behavior," Blank said.

The down-low phenomenon, in which supposedly straight men sleep with other men on the sly, has been the subject of anecdotal reports in recent years.

However, this is the first time health experts have scientific evidence of it, Blank said.

A separate study in the medical journal found that an increasing proportion of deaths among New Yorkers with AIDS stems from causes other than the deadly disease itself.

Deaths from cancer, heart disease and drug abuse rose by 33% among the city’s AIDS patients between 1999 and 2004.

However, three-quarters of New Yorkers with AIDS still die of HIV-related illnesses.

"It’s the new drugs that are allowing people with AIDS to live longer and then die of other causes that kill the rest of us," said Judith Sackoff, city deputy director of HIV epidemiology.