Former US president George HW Bush has been admitted to an intensive care unit in Houston, his spokesman has confirmed.
The elder statesman has suffered a "series of setbacks including a persistent fever", but is alert and talking to medical staff, says his representative.
No further details were released about his medical condition, but doctors are cautiously optimistic about his treatment and the former US leader "remains in guarded condition" and is surrounded by his family.
The 88-year-old was admitted to hospital on November 23 to be treated for a lingering cough related to bronchitis. Previously, Mr Bush had been in and out of hospital for complications related to the illness.
Mr Bush's spokesman Jim Grath said on Wednesday that a fever which had kept his client in hospital over Christmas had become worse and doctors had put him on a liquid-only diet - but the cough that had initially been the problem had improved.
The statesman was visited on Christmas Day by family including his wife, Barbara, son, Neil, and Neil's wife Maria, and a grandson, Mr Bush's daughter, Dorothy, was expected to arrive on Wednesday in Houston from Bethesda, Maryland.
The former president has also been visited twice by his sons, former president George W Bush, and Jeb Bush, the former governor of Florida.