Stepanek died at Children's National Medical Center in Washington, the hospital said. He had been hospitalized since early March for complications related to the disease that impaired most of his body's functions.

In his short life, the tireless Stepanek wrote five volumes of poetry that sold millions of copies. Three of the volumes reached the New York Times' best-seller list.

"Mattie was something special, something very special," entertainer Jerry Lewis, who chairs the Muscular Dystrophy Association, said in a statement.

"His example made people want to reach for the best within themselves."

Stepanek, of Rockville, had dysautonomic mitochondrial myopathy, a genetic disease that impaired his heart rate, breathing, blood pressure and digestion, and caused muscle weakness.

His mother, Jeni, 44, has the adult-onset form of the disease, and his three older siblings died of it in early childhood.

From deathbed to huge publishing success

Stepanek began writing poetry at age 3 to cope with the death of a brother. In 2001, a small publisher issued a slim volume of his poems, called "Heartsongs." Within weeks, the book reached the top of the Times' best-seller list, the MDA said.

He wrote four other books: "Journey Through Heartsongs," "Hope Through Heartsongs," "Celebrate Through Heartsongs" and "Loving Through Heartsongs."

His poems brought him admirers including Oprah Winfrey and former President Carter and made him one of the best-selling poets in recent years.

Stepanek was hospitalized many times over the years. He rolled around his home in a wheelchair he nicknamed "Slick," and relied on a feeding tube, a ventilator and frequent blood transfusions to stay alive.

In the summer of 2001, Stepanek nearly died from uncontrollable bleeding in his throat and spent five months at Children's National. When it seemed he would not survive, the hospital got in touch with a Virginia publisher on his behalf.

Stepanek and his mother had sent the book to dozens of New York publishers, all of whom rejected it, according to Peter Barnes of VSP Publishers. Barnes said he was caught off guard when he read the work.

"I was stunned. Some of it was really good," he said Tuesday. "It was very perceptive and thoughtful."

VSP Books printed 200 copies of "Heartsongs" to be handed out to friends. But after a news conference publicizing the book, interest exploded. "Heartsongs" went on to sell more than 500,000 copies.

"Mattie rallied after that," Barnes said. "He went from being on his death bed to becoming this huge publishing success."

Despite his condition, Stepanek was upbeat, saying he didn't fear death. His work was full of life, a quest for peace, hope and the inner voice he called a "heartsong."

"It's our inner beauty, our message, the songs in our hearts," he said in an interview with The Associated Press in November 2001. "My life mission is to spread peace to the world."

He is survived by his mother.

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This is sad to hear.I was watching a interview he did with Larry King earlier and he seemed like a nice kid.I can relate to him in alot of ways.I too was close to death when I was born.I was born pre mature and my lungs weren't fully developed.I felt sorry for him when he was talking about one his brothers dying.He said that his parents didn't want him to go in his brothers room.He went in there when his parents weren't around and he saw them putting his brother in a casket.At first he thought he was playing hide and seek and at the funeral he started knocking on his casket too see if he was still playing or not.Mattie said when he saw them put his brother in the ground that's when he found out he was gone.

He reminds me of myself in some ways.With his writing and wiseness.I hope he is happy in the after life knowing he helped people's life's.After seeing that interview it made me want to change my life around.Lately I've been pretty pissed off.Alot of stuff I wrote in my notebook has been about death and the hard times of life.Im slowly starting to get over it.I don't want to try taking that suicide route like I attempted to do before when I was 11.Im just going to deal with all the BS people give me.And keep looking up.When bad days are in front of you good days are not too far away.