Rory McIlroy quits: McIlroy quits Honda Classic after dropping seven shots
Before driving away from the course, defending champion McIlroy reporters: "I'm not in a great place mentally. I can't really say much, guys. I'm just in a bad place mentally."
In a statement released later, the world number one said he had been suffering with severe wisdom tooth pain.
"It began bothering me again last night," said McIlroy. "It was very painful again this morning, and I was simply unable to concentrate. It was really bothering me and had begun to affect my playing partners."
McIlroy shot an even-par 70 in Thursday's opening round at PGA National but had a nightmare start on Friday that included a triple-bogey, a double-bogey and two bogeys.
After eating a sandwich, he hit his approach shot into the water on the 18th, his ninth hole of the day, and promptly walked off the course with his coach and caddie.
McIlroy would almost certainly have missed the cut and his decision to quit was criticised by playing partner Ernie Els who said: "I'm a great fan of Rory's but I don't think that was the right thing to do."
Els later said McIlroy deserved credit for playing through pain while denying that the 23-year-old Northern Irishman's discomfort had been affecting his partners.
"All credit to him but no. I've played like that before. It's embarrassing. You don't want to be out there while you feel like, get me out here," said the South African, the current British Open champion.
'REGRET DECISION'
McIlroy has played just four full professional rounds in three months, missing the cut in the Abu Dhabi Championship in January and suffering a surprise first-round exit in the WGC-Accenture World Match Play Championship last week in Arizona.
In January, the 23-year-old switched his club brand from Titleist to Nike in a lucrative deal reported to be worth as much as $250 million over 10 years.
Former world number one Nick Faldo called the change in club manufacturers "dangerous" and the concerns have hardly been eased given the two-times major winner's slow start to the year.
Prior the this week's tournament, McIlroy said he was still in "an adjustment period" but stressed he was more concerned with his swing than the feel of new clubs.
Beginning play on the 10th hole, McIlroy struggled early. He nearly hit his approach to the par-four 11th hole into the water before chipping through the green and making a double-bogey.
Two holes later, he made a bogey after driving to the right and being forced to lay up. He missed a six-footer to save par.
On the par-four 16th, McIlroy hit his tee shot into the water, took a drop, then hit another into the water before carding a triple-bogey seven. A three-putt on the par-three 17th left him seven-over par after eight holes.
"I came here with every intention of defending my Honda Classic title. Even though my results haven't revealed it, I really felt like I was rounding a corner," said McIlroy.
"This is one of my favorite tournaments of the year and I regret having to make the decision to withdraw, but it was one I had to make."
