NEW YORK — Peter Jennings, anchor of ABC News' "World News Tonight" for more than 20 years, has been diagnosed with lung cancer, he said yesterday.

Jennings, 66, is scheduled to begin chemotherapy treatments next week in New York, but plans to continue anchoring the news program as long as he is able.

ABC News President David Westin said that Charles Gibson, Elizabeth Vargas and others would substitute for Jennings "from time to time."

Noticeably thinner and with a hoarse voice, Jennings told viewers near the end of last night's newscast, anchored by Vargas.

The news adds to the tumult caused by Ted Koppel's announcement last week that he would end 25 years as the host of "Nightline," the network's late-night news show.

In an e-mail, Jennings told his senior staff that he had been diagnosed with lung cancer Monday.

"Dear All — Forgive me the group mailing — but it seems the easiest way to tell a lot of people I care for ... about a change in my life," he wrote. "I have been diagnosed with lung cancer. Yes, it was quite a surprise."

Jennings added: "There will be good days and bad, which means that some days I may be cranky and some days really cranky! 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."

Lung cancer is the biggest cancer killer, affecting an estimated 172,570 Americans and killing more than 163,000 annually. About 42 percent of patients live a year, but only 15 percent are alive five years later.

Jennings is a former smoker who quit several years ago.

Although the announcement stunned ABC staffers, it did not come as a complete surprise, said Judy Muller, a contributing correspondent to "World News Tonight" and an assistant professor at the University of Southern California's Annenberg School for Communication.

Jennings, who championed international news coverage and was known for his stamina, was unable to cover the December tsunami disaster because of an upper-respiratory infection. When Pope John Paul II died Saturday, Jennings did not assume the anchor chair, typical during a major news event.

"People are so accustomed to Peter going everywhere that they were so surprised when he was too ill to cover the tsunami," Muller said. "Today, people are just stunned and sorry and hoping he gets better."

News of Jennings' illness comes a month after Dan Rather departed as anchor on the "CBS Evening News" and four months since Tom Brokaw left top-rated "NBC Nightly News."

A Canadian by birth, Jennings led the ABC nightly newscast from 1965 to 1967, making him the youngest network anchor in TV history. He took over ABC's Middle East bureau in Beirut in the early 1970s.

By 1983, Jennings was appointed sole anchor of "World News Tonight" as the program moved from London to New York.

Statistics on lung cancer were provided by Reuters.