Heinz Kerry, 74, was flown to Massachusetts General Hospital in Boston last night after doctors at Nantucket Cottage Hospital stabilised her, Glen Johnson, Kerry's personal spokesman said.
She became ill during a family holiday in Nantucket, US media reports said.
Kerry was with his wife as an ambulance first transported her to the island hospital, and also during her transfer to the Boston facility.
Brown said Heinz Kerry arrived in critical condition, although doctors were able to stabilise her. But neither the family nor hospital officials had released any more details about her medical emergency or her condition last night.
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"The family is grateful for the outpouring of support it has received and aware of the interest in her condition, but they ask for privacy at this time," Johnson said.
Heinz Kerry is the widow of former Senator John Heinz, and heir to the Heinz ketchup fortune, estimated in 2004 at USD 500 million. Heinz died in April 1991 when a helicopter crash.
In December 2009, doctors treated her for breast cancer.