SAN FRANCISCO -- Patients at University of California San Francisco Medical Center got a visit from the world's smallest horse Tuesday, NBC11's Traci Grant reported.

At 17 inches tall, weighing 57 pounds, Thumbelina is listed in the Guinness Book Of World Records as the world's smallest horse.

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"Both her parents were minis, and she just happened to be a dwarf, in addition to being a mini. So she's a mini, mini," said Michael Goessling of Goose Creek Farms.

Goessling said Thumbelina visited the medical center for an hour Tuesday to deliver smiles to sick children. The children's' playroom was packed with surgeons, nurses and media, all battling for the tiniest glimpse, Grant reported.

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"We had Barry Bonds here a few weeks ago, and he's a big hit with the kids. I think the horse is actually tying him, coming in very close, maybe even exceeding him a little bit," said Roxanne Fernandes of the UCSF Medical Center.

For parents, the strangest thing in the room wasn't the miniature horse -- it was seeing so many happy faces in such a tough place, Grant reported.

"It makes a lot of difference," said mother of a patient, Mala Ananthanurthy. "Usually the hospital is a dreary experience for the kids, and then they come here and they're so excited."

Thumbelina is on a tour of hospitals across the country. In addition to visiting children in 48 states, she's raising a million dollars for charity.