Stephen Rex Brown The Cabrini Center at 542 East Fifth Street.

The six-story building that houses a medical center catering to the elderly is on the market, raising concerns that a new landlord will give low-income patients the boot before the center can build a new location.

Last night, Community Board 3 sounded the alarm on the possible closure of the Cabrini Center for Nursing and Rehabilitation, sending a formal letter to the lawyer of the mystery buyer of the building at Avenue B and Fifth Street.

“Losing this facility would be a serious threat to health care services in Community Board 3,” the letter read. “The need for nursing home beds locally is already dire, and we cannot sustain a loss of the 240 beds at C.C.N.R., the largest facility in this community.”

The letter noted that the Cabrini Center had secured a new location at a site owned by the Archdiocese of New York, but that it would take five years to build. In the meantime, the center, which provides short-term rehabilitation and senior housing, could be left without a home and lose its operating license from the state.

“Thus, it is imperative that C.C.N.R. be permitted to stay past the expiration of its current lease in April, 2012,” the letter concluded.

Patricia Krasnausky, the president of Cabrini Eldercare, said the sale of the building at 542 East Fifth Street to the would-be buyer is not definite.

“We have no idea who they are,” Ms. Krasnausky said. “We want the [buyer] to know there are people living here that need a home and need taking care of. We want them to look at it in that perspective rather than as a real estate development.”

But one thing is for certain: the nameless buyer of the building has the politically connected Ken Fisher as his or her lawyer. Mr. Fisher, a former city councilman and former chair of the Land Use, Zoning & Planning Committee of the New York City Bar Association, has previously counted major developers like David and Jane Walentas as clients. Mr. Fisher would not comment on the concern surrounding the Cabrini Center.

Ms. Krasnausky said the center is consistently full and that it would take a long time for other facilities in Brooklyn and Queens to absorb its patients.

“Some of these people grew up in the neighborhood and would be moving out for the first time,” said Ms. Krasnausky, who would not reveal the center’s future location.

The anxiety over the future of the Cabrini Center comes a day after the Bialystoker Center on East Broadway closed due to financial woes.

The Archdiocese of New York was closed today and could not be reached for comment.

This article has been revised to reflect the following correction:

Correction: November 3, 2011

An earlier version of this article misstated the date of Bialystoker Center’s closing. It closed Oct. 31, one day before the publication of this item, not two months before.