BOSTON (AP) — The Run for the Hoses. The Duel in the Sun. The Inferno.

As the prospect of 80-degree temperatures looms over today’s Boston Marathon, race organizers are hoping the heat will forge a classic contest to rank among the legends of the event’s 116-year history but preparing for a potential medical crisis from runners wilting under a scorching sun.

The forecast forced organizers to offer a largely unprecedented deferment to the field of 27,000 that had spent the last year qualifying, registering and training for what is often a once-in-a-lifetime experience. The Boston Athletic Association also offered a deferment in 2010, when the Icelandic volcano eruption stalled air traffic in Europe and prevented about 300 runners from making getting to Boston.

There is no way of knowing how many will take them up on the offer this year until this morning, when the number of no-shows is calculated. But B.A.A. medical director Pierre d’Hemecourt warned runners with underlying medical issues, such as a cough or a cold or a recent stomach virus that left them dehydrated: “Please don’t run the marathon on Monday.”

Said B.A.A. executive director Tom Grilk: “We don’t want people to feel they have to run, because perhaps it’s not the wisest decision under these conditions.”

One year after cool temperatures and a significant tailwind — perfect running weather — helped Geoffrey Mutai finish in 2 hours, 3 minutes, 2 seconds for the fastest marathon ever, the heat has elite runners preparing for a slower pace and the recreational runners trying to figure out how to finish at all.

Although Mutai said he has never run a hot marathon, fellow Kenyan and defending women’s champion Caroline Kilel said she was used to training in warm weather. But Kilel said the typical conditions were more like 73 degrees — hot for a marathoner, but not quite what’s expected today.

“The heat affects everybody. Nobody runs fast in the heat. Nobody benefits from the heat,” said 1968 winner Amby Burfoot. “If anyone’s been training in Miami, that would be great for them.”

The Boston Marathon has had its share of hot weather, with the thermometer hitting 97 degrees during the 1909 race that came to be known as “The Inferno” and the 1976 “Run for the Hoses” that started in 100-degree heat and finished with spectators sprinkling winner Jack Fultz with garden hoses to cool him down.

“It’s just the worst thing a marathoner can face,” said Burfoot, who ran with Fultz for half of the ’76 race before fading in the heat. “I hate that the marathon is such a crapshoot. You train for 4-6 months, and the only thing that matters is the weather.”

There were only 2,188 entrants in the ’76 race, and still just 7,647 six years later when Alberto Salazar won the “Duel in the Sun” against Dick Beardsley.

With 27,000 competitors registered this year, the heat presents organizers with a new challenge. B.A.A. officials will provide extra water, and medical attention will be beefed up on the course and at the finish.

But d’Hemecourt warned runners to take care of themselves, too, drinking more water than they usually do and recognizing the symptoms of heat stroke: confusion, headaches, nausea, vomiting and excessive fatigue.

“Personal responsibility has to happen on Monday,” race director Dave McGillivray said.