"As you all know, this is a challenge," Jennings wrote. "I begin chemotherapy next week. I will continue to do the broadcast. There will be good days and bad, which means that some days I may be cranky and some days really cranky!"
Jennings will have the outpatient therapy in New York, the Houston Chronicle said. Charlie Gibson, Elizabeth Vargas and others will substitute for him as necessary.
"It's both Peter's and my expectation that he will anchor World News Tonight during the period of treatment to the extent he can do so comfortably; but, we should also expect him to be off the broadcast from time to time, depending on how he feels," said ABC News executive David Westin.
Jennings joined ABC in 1964 and has been sole anchor of the popular newscast since 1983.