They chewed gum, they giggled and they gave the audience a measure of their courage as they talked, in a manner as forthright as their disease is dreaded, about what it's like to be a teen-ager with cancer.

To be told she had cancer, said 18-year-old Jill Chapman of Bridgeton, Mo., was "a quickie lesson in growing up. It's death threatening, we all live with that, but in the end it's all about life. About living, not existing."

Although the facts of their disease are ones that the audience of more than 300 parents and medical professionals are familiar with as an everyday part of their lives, the tales told with shy smiles and voices that haven't yet matured riveted listeners to the edges of their chairs in Marymount College in Arlington.

"I'm 16 years old," said Janet Lanigan who began the groups recitation of trials and triumphs. "I'm from St. Charles, Mo., I have osteosarcoma. My right leg was amputated in March of 1975." And in June of this year, she said, she was told that she has had a recurrence of the disease.

"My name is Jill Chapman, I'm 19, and I've had Hodgkins disease since January of 1974," said the young blond woman sitting near her. "I've had two recurrences." In July, she said, she will go off chemotherapy.

One by one, they talked quietly of the chemotherapy and the false limbs, the friends who supported them and the ones who did not, the terrors and even, occasionally, the small ironies that swept a quick breath of laughter through the audience.

They answered the audience's questions in the casual slang of adolescence and it was hard at times to connect the depth to which they had plumbed fear and the future's eclipse with their youthfulness.

"When they told me I had Hodgkins disease, I just said okay," Jill Chapman remembered. "But they never said the word cancer. I guess it sounds kinda dumb, but I just didn't catch on. Then one day I got out the encyclopedia and was reading up on this disease I had and I saw the word cancer and I nearly fell over. It was six more months before anyone else sid the word."

The audience, which was attending the first annual conference of the Candlelighters' Foundation, a nationwide nonprofit organization helping families of cancer victims form a network of mutual support, asked the panel questions that might help them in their own efforts to cope.

And the teen-agers tried to explain to them what it was like to try to brave the usual traumas of adolescence despite the long shadow thrown across their lives.

"It's hard sometimes," said Valerie Nelcamp, an honor student at the University of Cincinnati who turned 19 yesterday. Her cancer is now in remission, but not before she sacrificed a leg to it. "I know young men, especially, really pay a lot of attention to looks and physical attractiveness. I wonder - will a boy accept me just the way I am? You hear them talking about how a girl's really built, or how she looks in a bathing suit and I hope they understand that they just amputated my leg, they didn't take my heart and soul and personality. I just don't know if they'll ever accept me."

For all of them there has been the trial of being very different at an age so often presided over by the pressure of the peer group.

They were not prepared, they said, for the reactions of some of their friends. "You tell people what's wrong, and it's kind of like a freaky story to them," said Jill Chapman. "Their eyes get all wide and it's a gory story they can get a kick out of. And all you want is people who will just listen. People (with cancer) don't want you to talk, they don't want to know what you have to say, they want someone who let them talk about what it's like."

"When I was little," said Debra Vehewald, who has had cancer since 1968 when she was 8, "the kids in school would come up to me and say 'what's wrong with you, why do you always sit?' People would make fun and try to pull off my wig (worn after she lost her hair from drugs used in the chemotherapy.) I just thought it was something I had to live with. But now I say my piece."

They answered all the questions - 15-year-old Ken Daniels, who is trying out as a field goal kicker for the high school football team despite the loss of a leg, and 16-year-old Nichols Quiroz, who has had cancer for nearly a third of his life and says that after two recurrences of the disease, he won't be afraid if it comes back a third time. Or 15-year-old Malcolm Schorlemer who got the audience laughing when he said that "sometimes it seems like the nurses refrigerate their hands before they touch you" and stopped them when he said his one recommendation was "that they should always tell you when there's going to be some pain" in a treatment.

But there was one question that was more difficult to deal with. "My 15-year-old daughter asked me, 'why me?'" was the question from an anonymous parent. "'Why do I have leukemia.' What do I say to her?"

"You say that you don't know why," was the answer from one of the doctors. "What else is there to say?"