ORLANDO, Fla. - For 20 years Donna Rhone struggled with her weight. But the turning point came last year when she went on a cruise with a girlfriend.

During an emergency drill, the passengers had to walk up several flights of stairs. At 285 pounds, Rhone struggled to make it up the stairs. Her heart pounded and she gasped for air. Several times, the 38-year-old medical-billing specialist asked her friend to wait so she could catch her breath.

"When we finally reached the top she looked at me, and I could see how afraid and concerned she was. I felt so ashamed of myself that I could not complete such a task without feeling like I wouldn't make it," says Rhone. "That was my wake-up call."

Not long after the cruise, Rhone joined a Weight Watchers group. She has lost 84 pounds and, she reports, still has 50 to go. "I am more than halfway to my goal weight, and that is the most wonderful feeling in the world," she says.

Talk to people who have lost a lot of weight, and you'll often discover that they have one defining moment when they realize that they've got to do something about the extra pounds.

For Karen Fleming, the tipping point occurred when her marriage of 20-plus years dissolved. And for Linda Kline, it was the sobering prediction of a doctor who warned that she could die at any time.

"Many people are concerned about their weight, but people who successfully lose weight often experience a personal intersection," says Diane Berry, an associate professor of nursing at the University of North Carolina, whose research focuses on weight loss.

Often, she says, the one defining moment, their tipping point, is the accumulation of many experiences, from humiliations, embarrassments or frightening medical news.

In a study of 20 women who had lost 10 percent of their body weight and kept it off for a year, Berry found that most of the women experienced a defining moment that led them to lose weight. For some, it was someone's critical comment about their appearance; for others it was stepping on a scale and being shocked at their weight or buying a larger dress size.

For Kline, her tipping point came when she saw a series of doctors about her sleep apnea. One doctor offered to put a hole in her throat so she could breathe at night. Another doctor told her to quit her job and go on disability. Another specialist was more blunt: At 400 pounds and suffering from sleep apnea, you are at high risk for sudden death.

To get rid of the sleep apnea, he told her she'd have to lose 200 pounds. Kline wanted to avoid fat farms and diets - all the things she'd tried before. "I would lose 100 pounds and then gain 150 back," she says. "That's why I was so frustrated."

Determined to lose weight permanently, Kline hired a personal trainer at the YMCA. Through the YMCA's Healthy Weigh program, she lost 220 pounds in 18 months. Now she works out every day, doing an hour of cardiovascular exercise, along with three days of weightlifting and three days of core and flexibility exercises.

"I never want to feel that way again," Kline says. "I had high blood pressure. I had neuropathy in my legs. I had diabetes. But now my diabetes and blood pressure are under control. I've gone from taking eight pills a day down to zero."

For Fleming, the turning point came last year, when she got divorced.

"Instead of celebrating my 25th wedding anniversary, I was in the process of going to a divorce court," she says.

During the divorce, Karen, who weighed 265 pounds, lost 15 pounds from the stress. Afterward, she gave herself a birthday present: She joined Weight Watchers.

"For years, I had always put my husband first," says Fleming, 48. "When it was over, I thought, 'It's time to think about myself.'"

Now, a year after she joined Weight Watchers, Fleming has lost almost 70 pounds and plans to lose another 50 pounds. She has joined a gym and that, combined with her job as kennel manager for a greyhound rescue group, has helped her go from size 30 clothes to a size 16.

Says Carol Ann Mekker, her Weight Watchers team leader: "That sleek Greyhound body will be hers before she knows it."