Chief ABC anchorman Peter Jennings (search) revealed Tuesday that he is suffering from lung cancer and plans to continue on "World News Tonight" as much as possible after beginning chemotherapy next week.

Jennings was replaced by Elizabeth Vargas (search) on Tuesday but taped a message telling viewers about his diagnosis.

"I will continue to do the broadcast," he said. "On good days, my voice will not always be like this. Certainly, it's been a long time. And I hope it goes without saying that a journalist who doesn't value — deeply — the audience's loyalty should be in another line of work."

Jennings wrote in a memo to his staff: "I have been diagnosed with lung cancer. Yes, it was quite a surprise ... As you all know, this is a challenge. I begin chemotherapy next week."

ABC News president David Westin (search) addressed staffers in another memo:

"This morning, Peter Jennings told his senior staff at 'World News Tonight' that yesterday afternoon he was diagnosed with lung cancer," Westin wrote. "All of us at ABC News have watched over the years as Peter has led us on various assignments with strength and with courage. We've done our best to support him in these endeavors.

"Now, Peter's been given a tough assignment. 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. I know that all of us will give him every bit of support that he needs and asks for. Peter will once again lead the way, but we will stand with him at every turn."

In a 1996 Los Angeles Daily News article, Jennings confessed that he had started smoking when he was 13 or 14, "using tea leaves wrapped up in toilet paper."

He moved on to cigarettes and remained a heavy smoker until his children said they wouldn't let him in his house unless he quit.

Dr. Janice P. Dutcher, a professor at New York Medical College, told FOX News that serious damage can sometimes already be done even if a person quits smoking.

"We don't know when the cells turn malignant," she said, adding that the disease is survivable.

"Most who are cured are cured from surgery," Dutcher said. "Sometimes we do [chemotherapy] first, [so the] tumor is more amenable, and do surgery after that."

Lung cancer is the leading cancer killer in the United States, and roughly four out of five people diagnosed with the disease die within five years, said Dr. Cliff Connery, chief of thoracic surgery at St. Luke's-Roosevelt Hospital in Manhattan.

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."

There are effective ways to treat lung cancer, but its mortality rate is so high because so many patients aren't diagnosed until their disease is in an advanced stage, Connery said.

Jennings had been feeling ill for the past several months and was replaced Saturday as anchor for coverage of Pope John Paul II's death by Charles Woodruff (search).

Nor did he travel to the area devastated by the Indian Ocean tsunami in December. ABC News said at the time that Jennings had an upper respiratory infection and was under doctor's orders not to travel. He did go to Iraq in January for that country's elections.

Jennings last anchored "World News Tonight" on Friday. He informed ABC News staff of the diagnosis Tuesday morning and said he would anchor the broadcast when he feels up to it over the next few months.

"There will be good days and bad, which means some days I may be cranky and some days really cranky," he told ABC News employees in the e-mailed memo. "Almost 10 million Americans are living with cancer. I am sure I will learn from them how to cope with the facts of life that none of us anticipated."

Charles Gibson (search), in Rome for the papal funeral, and Vargas will be Jennings' primary substitutes on "World News Tonight."

Jennings first anchored ABC's evening newscast for two years in the 1960s, and after that he was a Europe-based correspondent for the network, finally heading the London bureau before coming back to New York.

He was an anchor again when ABC went to a multi-anchor format in 1978. ABC abandoned the approach in 1983 after Frank Reynolds (search) died of cancer. Jennings has been the sole anchor since then.

"World News Tonight" dominated the ratings during the late 1980s and early 1990s before being surpassed by NBC's Tom Brokaw (search). The broadcast is now a close second to "NBC Nightly News," currently anchored by Brokaw's successor, Brian Williams (search).

Jennings is the last of the troika of anchormen who dominated broadcast network news for the past two decades. Brokaw stepped down last year and Dan Rather (search) left the "CBS Evening News" news last month.

"Peter is an old friend," Brokaw said Tuesday. "I'm heartbroken, but he's also a tough guy. I'm counting on him getting through this very difficult passage."

Others took the opportunity to praise Jennings' on-air style.

"Jennings has served American broadcast news with a graceful internationalist intelligence," said Rich Hanley, director of graduate programs at Quinnipiac University's school of communications.

"His manner of news delivery stood in contrast to but not in opposition of the gentle Midwestern aesthetic expressed by Tom Brokaw and the don't-mess-with-Texas eccentricity of Dan Rather."

Jennings was born and raised in Toronto but proudly became a U.S. citizen in 2003. He resides in Manhattan with his wife, Kayce Freed. He has two children.

The Associated Press contributed to this report.