Matthew Joseph Thaddeus Stepanek, 13, the cheerful, bespectacled child poet who charmed Oprah and sold more than 500,000 books of dreamy verse, died yesterday at Children's Hospital in Washington. He had a rare form of muscular dystrophy that affected his breathing, digestion and heart rate.

Mattie, who gained international acclaim as a poet and a peace advocate, used his neuromuscular disease to raise awareness of muscular dystrophy. The disease, dysautonomic mitochondrial myopathy, forced the Rockville youth to use a wheelchair and made him dependent upon a ventilator, supplemental oxygen and blood transfusions.

But the young poet's effervescent philosophy, which he summarized as "remember to play after every storm," attracted the attention of television talk show hosts Larry King and Oprah Winfrey and thousands of other fans.

Publisher Peter Barnes said his company received thousands of letters addressed to Mattie in recent years. One, he said, came from a man with AIDS who felt suicidal until he read the poems, which he said saved his life.

Mattie described himself on his Web site as a practical joker who had earned a first-degree black belt in the martial art hapkido before he had to use a wheelchair. He also said that sometimes he got angry and scared.

"I am very human. Some people think I am always brave. I try to be, but I cry like the next person sometimes. I am needle phobic and pain phobic, so that doesn't help," he said on the Web site. "But even if I get upset, or think, 'I can't do this anymore,' I get myself together and pray or play or talk with my mom or a close friend, and I get beyond that tough time. I might say, 'Why me?' But then I say, 'Why not me? Better me than a little baby, or a kid who doesn't have strength or support.' "

Mattie began reciting poems at 3, before he could write. His mother, Jeni Stepanek, who has the adult-onset form of the disease and who lost her three older children to it, took notes.

He told Larry King in February 2003 that he wrote almost every day. "How my mom describes it is I'm like a volcano," he said. "I either do nothing, thinking about when I'm going to do it, or I just burst, spurt out everything."

His first book of poems, "Heartsongs," was published in 2001 by VSP Publishers, a small Virginia publisher, and within weeks it shot to the top of the New York Times bestseller list, helped by his appearances on "Larry King Live," "Oprah" and "Good Morning America" and by an article in People magazine.

Four more books of poetry followed: "Journey Through Heartsongs," "Hope Through Heartsongs," "Celebrate Through Heartsongs" and "Loving Through Heartsongs." Some of his poems were set to music and released as an album of songs 14 months ago by Billy Gilman.

Mattie became the Muscular Dystrophy Association's national goodwill ambassador from 2002 until he died, and appeared on the "Jerry Lewis MDA Telethon" three times. Lewis said yesterday: "His example made people want to reach for the best within themselves. It was easy to forget how sick he was because his megawatt personality just made you want to smile."

Dysautonomic mitochondrial myopathy is one of 43 different illnesses that are called muscular dystrophy. Although fewer than 1,000 people in the United States have that particular disease, almost a million people have muscular dystrophy diseases.

Mattie attended school until he was 9 and then was home-schooled. At 11, he wrote on the "My Heroes" Web site: "When I grow up, I want to be a peacemaker. My biggest role model for this is Jimmy Carter, who has been a wonderful peanut farmer, politician, and peacemaker. . . . I call him the 'perfect hero.' "

Mattie Stepanek, who had a rare form of muscular dystrophy, used his illness to raise public awareness. Mattie with his mother, Jeni, also a muscular dystrophy sufferer.