Now they've learned the sad reason why: Jennings has lung cancer.
Jennings broke the news in an e-mail to colleagues Tuesday and, in a taped message because his husky voice wouldn't permit him to anchor "World News Tonight" in the evening, to his viewers.
"Almost 10 million Americans are already living with cancer, and I have a lot to learn from them," he said. "And living is the key word."
But lung cancer is the nation's deadliest cancer, says CBS News Correspondent Dr. Emily Senay.
"This is a cancer that's not diagnosed in the earliest stages when you can literally cut it out with surgery," Senay said on CBS News' The Early Show. "Most people who have lung cancer, when it's found, it is advanced."
Jennings, who begins chemotherapy treatment next week, said he intends to keep anchoring the evening news. But he acknowledged there will be days when he's not up to it.
For two decades Jennings, NBC's Tom Brokaw and CBS's Dan Rather were the anchors who were always there, telling Americans about the Reagan administration, the fall of communism, the O.J. Simpson trial, the Sept. 11, 2001, terrorist attacks and countless other stories.
Now, with almost blinding speed, that era is over. Brokaw left last November, replaced by Brian Williams. Embattled Rather said goodbye to The Evening News last month.
And Jennings, who had looked forward to a period as the business' elder statesman, is fighting for his health.
More than 170,000 cases of lung cancer are diagnosed in the U.S. each year, and more than 160,000 deaths result — more than other more-talked-about forms of cancer combined, says Senay.
"It's also an aggressive cancer. It grows quickly," she said.
Eighty-five to 90 percent of the cases are related to smoking, Senay added.
"Yes, I was a smoker until about 20 years ago," Jennings said, "and I was weak and I smoked over 9/11. But whatever the reason, the news does slow you down a bit."
Doctors said most lung cancer patients can continue to work throughout treatment, but need flexibility to take it easy on days they are not feeling well.
With his very visible position on television each night, Jennings could be an inspiration for many Americans going through a similar fight, said Dr. David Johnson, chief of oncology and hematology at the Vanderbilt University School of Medicine.
"I think it sets the right example," Johnson said. "I think it says you shouldn't stop your life if you have cancer. It may take your life, but you shouldn't let it control your life."
"He's already bringing to this new challenge the courage and strength we've seen so often in his reporting from the field and in anchoring ABC News," Westin told ABC staffers. "I know that all of us will give him every bit of support that he needs and asks for."
While still in his 20s, Jennings anchored ABC's evening news for two years in the 1960s. He returned to the desk in 1978 when third-place ABC tried a multi-anchor format, which was abandoned in 1983 when Frank Reynolds died from cancer. Jennings has been ABC's sole evening anchor ever since.
A Canadian who proudly became a U.S. citizen in 2003, the urbane Jennings dominated the ratings from the late 1980s to the mid-'90s, when Brokaw surpassed him.
Lately, Jennings has been inching up in the ratings against Brokaw's replacement, Brian Williams, who sent along best wishes on Tuesday. "Peter is a friend and the best of competitors," Williams said. "It will take more than this to scare the man I know."
Senay said all long-time smokers should consult with their doctors about the possibility of lung cancer.
"If you have symptoms such as infections that don't go away, shortness of breath, chest pain when you take a deep breath, if you're coughing up blood, or generalized things such as fatigue or weight loss, and you have a history of smoking, these are all good reasons to check it out with your doctor," she said.