War of Will has bounced back from a bumpy ride in the Kentucky Derby to win the Preakness Stakes at Pimlico, holding off a field that included a riderless horse which threw his jockey out of the gate.

Trainer Mark Casse celebrated his first Triple Crown victory with War of Will who suffered interference in the Kentucky Derby, which led to first-place finisher Maximum Security being disqualified.

Bodexpress threw his jockey John Velazquez just out of the starting gate but still finished the race without creating any problems.

War of Will made a move around the final turn under Tyler Gaffalione and did not relent down the straight. Hard-charging late addition Everfast came in second and Owendale third.

An inquiry was briefly put up on the board at Pimlico Race Course but quickly taken down.

Casse, 58, entered a horse in the Preakness for the fifth time and came closest two years ago when Classic Empire finished second.

The trainer was just thankful War of Will did not go down in the Derby, which could have been a multi-horse catastrophe.

"This is even I think probably more special given everything that we've been through," Casse said.

"I'm not even calling it redemption. I didn't feel like he got his fair shot, and that's all I wanted - a fair shot. And he showed what he had today."

It's also a breakthrough for 24-year-old Gaffalione, who has become something of a rising star since being named top apprentice rider in 2015.

The Bob Baffert-trained Improbable was beaten as the favourite for the second consecutive Triple Crown race.

Neither Maximum Security nor Kentucky Derby winner Country House contested the Preakness.

Velazquez did not seek medical attention after being tossed from Bodexpress.

"He wasn't behaving well," Velazquez said.

"When the doors opened, I was off right from the start and he kind of jumped sideways, and I had my feet out sideways and I lost my balance and went out."

The race was run at a tumultuous time for American horse racing.

After 23 horse fatalities at Santa Anita Park over a three-month span, there was another in training on Friday, and a filly collapsed and died after a race at Pimlico on Friday. The disqualification of Maximum Security at Churchill Downs has prompted a lawsuit by owners Gary and Mary West and a suspension for jockey Luis Saez.

In Maryland, the Stronach Group that owns the track - and also Santa Anita - is embroiled in an ongoing quarrel with local politicians over the future of the Preakness being at historic but ageing Pimlico or the owners' favoured Laurel Park about 30 miles south.

More than 6000 grandstand seats were cordoned off because they were deemed unsafe, and a water main break disrupted preparations for the event and left a lack of running water on race day.