"Larry has fought so many health issues in the last few years and he is fighting this one hard too. He's a champ," the source said.
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The 87-year-old former television host is hospitalized at L.A.'s Cedars-Sinai Medical Center. Details of his condition were not immediately disclosed.
Hospital protocols have kept King's family members from visiting him.
The Peabody Award-winning broadcaster was among America's most prominent interviewers of celebrities, presidents and other newsmakers during a half-century career that included 25 years with a nightly show on CNN.
He has had medical issues in recent decades, including heart attacks and diagnoses of diabetes and lung cancer.
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Last year, King lost two of his five children within weeks of each other. Son Andy King died of a heart attack at 65 in August, and daughter Chaia King died from lung cancer at 51 in July, Larry King said then in a statement.
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The Associated Press contributed to this report.