"After conducting tests overnight and this morning, doctors at Massachusetts General Hospital have upgraded Mrs Teresa Heinz Kerry's condition from critical to fair," said Glen Johnson, personal spokesman for John Kerry.
"She is undergoing further evaluation, and Secretary of State Kerry, her son, and other family members remain with Mrs Heinz Kerry at the hospital in Boston, as they have been since she became ill," Johnson said.
She arrived in the hospital in a critical condition, but the doctors were able to stabilise her condition later.
Heinz Kerry is the widow of former Senator John Heinz, and heir to the Heinz ketchup fortune, valued in 2004 at USD 500 million.
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John Heinz died in April 1991 in a helicopter crash.
She married John Kerry in 1995 and was previously treated for breast cancer.
Kerry, who is scheduled to co-chair the US-China Strategic and Economic Dialogue later this week remains in Boston due to his wife's illness.