“Now,” Robin Roberts said to the staff of her top-rated morning show, “Good Morning America,” right after it wrapped on Wednesday, “we can resume regular programming.”

Ms. Roberts had just made a television comeback unlike any other, as a host of the program for the first time since she was forced to leave it in August to fight a life-threatening illness. The return, promoted two weeks ahead of time by ABC, was celebrated by fans, tens of thousands of whom sent well-wishes on social networking sites. Many of them watch the program specifically for Ms. Roberts, who is, according to industry research, the most-liked host on any American morning news program by a wide margin.

On Wednesday it was obvious why. She beamed with pride and gratitude as she returned to the broadcast, defying the expectations of some in the television industry who had predicted she would be unwilling or unable to anchor again.

The broadcast gave ABC fresh optimism that “Good Morning America,” with Ms. Roberts, 52, back in her chair, can continue to beat NBC’s “Today,” which last year was dislodged from the top spot in the morning ratings after 16 straight years.

Most of all it closed a chapter in a story that started almost exactly one year ago, when Ms. Roberts felt exhausted while covering the 2012 Academy Awards in Los Angeles for ABC. Subsequent tests by her doctors found that she had myelodysplastic syndromes, known as M.D.S., a rare and debilitating blood disorder, most likely resulting from her treatment for breast cancer five years earlier.

After taking a leave in August she underwent a bone-marrow transplant in September. Back then Ms. Roberts told viewers she’d return to “Good Morning America” as soon as she could. But no one knew for sure how long she would be away, if she survived at all. Nor could anyone at ABC think of any precedents for a leave like hers. “There was no handbook for this, but I’m very pleased with how we handled it,” Ms. Roberts said in an interview after Wednesday’s broadcast.

The closest things to precedents were weeks-long maternity leaves, and the one thing ABC was determined not to repeat: a departure like that of Peter Jennings, the longtime “World News Tonight” anchor who abruptly came onto his newscast one day in April 2005, announced he had lung cancer, said, “I will continue to do the broadcast,” and never came back.

Jennings died four months after making the announcement, and the circumstances were traumatic for viewers as well as for ABC staff members. For that reason — as well as for the more obvious ones involving ratings and reputation — ABC decided to make Ms. Roberts a part of the show even while she was in the hospital recuperating from the transplant.

George Stephanopoulos and the other co-hosts mentioned her by name at least once every half-hour, and they shared her Twitter messages and photos on TV regularly. On Wednesday everyone involved emphasized that she was returning now only because her doctors say she is ready.

Nonetheless morning TV is big business, so there were grumblings that ABC had exploited her condition for ratings gains. Last July, two weeks after NBC removed Ann Curry from “Today,”spurring a big lift in the ratings for “Good Morning America,” the “Today” executive producer Jim Bell wrote in an e-mail to senior producers that the competition was “using Robin’s illness and the accompanying public interest in her health as a new weapon in its arsenal.”

More recently some media critics have censured the show for overcovering Ms. Roberts’s impending return. But if online chatter is any indication, that sentiment wasn’t widely shared by the viewers who have been rooting for Ms. Roberts and for her television family, which remained firmly No. 1 among total viewers while she was away. But among the 25- to 54-year-old viewers that help the shows make money, “Good Morning America” stayed only slightly ahead of “Today,” and within ABC, there is a quiet hope that Ms. Roberts’ return will propel the program forward.

Ben Sherwood, the president of ABC News, ducked questions about the ratings on Wednesday but said, “This experience has reminded us to take nothing for granted, and, like Robin herself, in many ways we feel like we’re just getting started.”

He also said, “After 173 very long days, it’s beautiful to get back to business as usual with our full team and two more wonderful regulars.” By “regulars” he meant Elizabeth Vargas and Amy Robach, who took turns filling in while Ms. Roberts was away. They will continue to show up frequently on the program, he said, in part because Ms. Roberts is not yet back at full strength. She’ll re-emerge gradually, for a few days a week at first, depending on how she and how her doctors feel about her progress. At least one thing is certain: she will be back in Los Angeles to cover the Academy Awards this weekend.

Mr. Sherwood, when asked if Ms. Roberts would ever return to a five-day-a-week schedule, observed that she’s an athlete first and foremost (a star college basketball player and former ESPN sportscaster) who “wants to play every day.” He added, “I think nothing would make Robin happier than getting back to five days a week.”

Even the most cynical “Good Morning” producers — who requested anonymity because they were not authorized by the network to speak — pointed out that Ms. Roberts’s story could have ended very differently. “It doesn’t matter about ratings” on Wednesday, one such producer said in between emotional expletives. “She is alive!”

Interviewed by People magazine, which put her on its cover last week, Ms. Roberts said she was warned that “at one point I would feel like dying.” Shortly after the transplant, that came true, she said: “I was in a pain I had never experienced before, physically and mentally. I was in a comalike state. I truly felt like I was slipping away. Then I kept hearing, ‘Robin! Robin!’ ” The voice belonged to a nurse, who was “pleading for me to stay here,” Ms. Roberts said. “And thankfully I did. I came back.”

Ms. Roberts said she didn’t feel especially tired after her two-hour hosting blitz on Wednesday. She did notice some problems with her vision, however, since she had not been under the bright studio lights for such an extended period since August.

Her return was even cause for a temporary cessation of hostilities between “Today” and “Good Morning America.” “Today” sent a gift basket to the ABC studio and welcomed Ms. Roberts back during its 8 a.m. hour.

Don Nash, who succeeded Mr. Bell as executive producer of “Today” two months ago, said in an e-mail on Tuesday night, “All of us at ‘Today’ wish her continued good health and years of hitting the 3 a.m. snooze button!”