A carbon monoxide leak sickened five employees at a tony Upper East Side restaurant owned by famous chef Jean-Georges Vongerichten Tuesday night and firefighters had to briefly evacuate the building, authorities said.

All five workers at JoJo were taken to the hospital and expected to be OK after an oil burner — which provides heat and hot water — malfunctioned in the basement, FDNY Chief Mark Rosenbaum said.

“We discovered the oil burner was not working properly. It was giving off high levels of CO throughout the basement,” Rosenbaum said. “There was no extension – it didn’t leave the basement.”

Employees and residents who live over French eatery on East 64th Street were evacuated at 11 p.m., about 30 minutes after closing time.

They were allowed back inside a little before 1 a.m.

Restaurant staff declined comment.