Speedo? Check. Beard? Check.

"One of the guys said, `We should do something crazy this year,' " recalls Jon Ianelli, and the Santa Speedo Run was born. That first year, it was just the five friends: Ianelli, Seth Frye, Dave Swanson, Nate Blew, and Ryan Birkenhead. They donned Speedos, sneakers, and Santa beards and hats and took off for a run down Newbury Street and back up Boylston.

"We were terrified," says Ianelli, 30. "But by the end, we all wanted to do it again. People were pulling out cameras and taking pictures." He adds: "I'll be honest with you. We started at a bar and had a couple of beers to get our courage up."

The next year, they put together a website (www.santaspeedorun.com), attracting about 30 male runners. They charged a $20 entry fee and chose the Sept. 11 Children's Fund as their charity. Last year, 50 guys ran, and the proceeds went to the Michael J. Fox Foundation for Parkinson's Research.

Mark Macklin was among the runners. Since he's Jewish, he billed himself as "the running rabbi." He wore a black Speedo, a black fake beard, and a black fedora and carried a gold menorah. For sidelocks, he used 3-inch pieces of an old telephone cord.

"It was 31 degrees and raining," he recalls, "but it was awesome."

This year, about 100 people have registered so far for the milelong run, which starts Saturday at 2 p.m. ("Meet at noon at Daisy Buchanan's for a few spirits to get your courage up," says Ianelli.) The guys sing Christmas carols along the way and do push-ups and jumping jacks at stoplights to stay warm.

"You don't have to be in really good shape," says Ianelli, suggesting that skeptics check out the photos on the website. "It's strength in numbers, so no one person is going to stand out." He notes that he's not a Speedo regular: "I only wear it for the run."

This year, Ianelli is asking each runner to obtain at least $100 in pledges. He's hoping to raise $10,000 for the Children's AIDS Program at Boston Medical Center. The program targets babies and children who have HIV or AIDS and offers free daycare, meals, after-school programs, camp, and teen services. Parents who are struggling with the disease receive support, too.

Barbara Farrell, program director, says she is thrilled. "This couldn't come at a better time," she says. The program urgently needs a new van to transport some of the children.

Ianelli, noting that many guys aren't great at fund-raising, has a tip: "You just ask 10 people for 10 bucks. It's a piece of cake." Last-minute entrants can show up at Daisy Buchanan's and pay $30 to run.

This year, for the first time, some women are running. Kim Huerth, 22, is wearing a black two-piece Speedo set. "I decided to go for it because it's not a very big loop we're running and it's for a good cause, and it's a fun thing and easy way to raise money," she says.

But she acknowledges she was loath at first to sign up. "It had more to do with a lack of confidence in my physical appearance than with any aversion to the cold. But then I realized that any discomfort I might incur . . . is nothing compared to the fear and pain that children with AIDS must face every day," says Huerth, a graduate student and substitute teacher.

She's been in training, sort of. "Every time I step outside, I try to imagine what it will be like in just a two-piece bathing suit," she says. "I'm just going to grit my teeth and bear it." That and have a beer at Daisy's beforehand.