PETER LOISELLE, 41

LOST 503 lbs.
Loiselle and his students with a pair of his old 84 inch-waist pants in their Ellsworth, Maine, classroom.
HIS SUMMER TIP: Bring your own light buns and rolls to barbecues.

Nicknamed "Big Pete" since the second grade, Loiselle was always the largest kid in class. At high school graduation he was 300 lbs. Twenty years later his weight had reached 763 lbs. and his waistline measured 84 inches. Still, "I was never dissatisfied with who I was," says Loiselle, a middle school math teacher who used to get winded walking down the hall. "But I had resigned myself to the fact that I was going to die young." When he learned in 2001 he would have to lose 100 lbs. to be considered for gastric bypass surgery, he decided to diet and exercise instead. He cut his calories from 5,000 a day to 2,500 and used an arm bike for 15 minutes at a time. Now 503 lbs. lighter, he walks four miles daily, lifts weights three times a week and stays on a low-calorie diet. "My lifestyle is so changed," he says. "Now I can go to the movies. I can get on a roller coaster. I can ride in a Ferrari!"

SISSY LUSK, 40

LOST 222 lbs.
For a summer snack, "I put half a banana on a Popsicle stick and freeze it." says Lusk (in Naples, Fla.).

Shedding two-thirds of her body weight wasn't easy, but Sissy Lusk had the help of her husband, John. "I could lose two-tenths of a pound one week and he'd say, 'Oh, honey, I can really tell in your hips!'" she says. Overweight since childhood, Lusk, who lives in Canton, Ga., had tried "the grapefruit diet, the milk diet, the hot dog diet," she says. "I'd just go off them after a couple of days and then go nuts eating." When she reached 345 lbs. in 1999, she began worrying that her daughter Elizabeth, now 12, might be teased at school. "A classmate once asked if I was pregnant," she says. So she began walking up to five miles a day and joined Weight Watchers. In three years Lusk lost 222 lbs.—and then became a group leader at Weight Watchers, which helps her stick to her own diet. "It's hard to forget what you're supposed to be doing," she says, "when you're teaching others to do it 11 times a week."

NICK YPHANTIDES, 39

LOST 267 lbs.
At summer pool parties, says Yphantides (at a baseball park in Escondido), "I spend more time in the water than around the food."

At a local diner in Escondido, Calif., Dr. Nick Yphantides effortlessly slides into a booth. "I can't believe I am sitting here," he says. "I couldn't fit before." Big Greek meals with his family helped him pack on 467 lbs. But a bout with testicular cancer at age 32 inspired him to make a change. "I dodged the cancer bullet, but I was killing myself with food," says Yphantides. In 2001 he went on a medically supervised liquid protein diet and distracted himself from eating with a nine-month road trip, attending baseball games in every major league ballpark and writing a book about it called My Big Fat Greek Diet. Says Yphantides, who now eats small portions of solid food: "I don't have to have food to have a good time."

SUSAN WALKUP, 32

LOST 105 lbs.
"I used to eat fatty meat like ribs at barbecues," says Walkup (in Tacoma). "Now I eat grilled fish and chicken."

When Susan Walkup was 229 lbs. and a size 26W, she was so ashamed of her body she hated to leave the house. "I hid at home," she says. Now a size 6, she helps keep her weight down by stepping out. She parks far from stores when shopping, and instead of e-mailing her colleagues at the mental health clinic in Tacoma, Wash., where she works, she walks to their offices. She even gets up to change the TV channel. Walkup decided to join Jenny Craig 3½ years ago, after a doctor warned her she would soon need medication for high blood pressure. "I have a family history of heart problems and knew I had to do something," she says. She reached her goal weight of 124 lbs. in a year and a half. Says her husband, Thane, who lost 40 lbs. himself: "She's an incredibly strong person."

Ericka Souter and Jennifer Wulff. Victoria Brett in Maine; Paige Bowers in Georgia; Vicki Sheff-Cahan and Sandra Marquez in California and Carolyn Campbell in Utah