No, an issue even more confounding: how he combs his hair.
Trump's hair, of course, has been the subject of endless fascination. A late-night television monologue wouldn't be complete without a joke about it. Is it real? And if it is, how does he achieve that gravity-defying look?
On Ryan Seacrest's radio show a year ago, Trump, 64, seemingly answered one of those questions. He let Seacrest's co-host, Lisa Foxx, run her fingers through his golden mane -- revealing a hairline that would be the envy of any man eligible for an AARP card.
"It may not be beautiful, but it's my own," Trump said in a rare bit of modesty. A video of the revelation, posted on YouTube, has been viewed more than 700,000 times.
Still, the fascination endures. On its website, Vanity Fair recently posted, "Donald Trump's Hair: The 360-Degree Tour," an examination of the mogul's head from every possible angle.
And at the recent White House Correspondents Dinner, Seth Meyers of "Saturday Night Live" got some of his biggest laughs when he took on The Donald's 'do.
"Donald Trump often appears on Fox, which is ironic because a fox often appears on Donald Trump's head. If you're at the Washington Post table with Trump and you can't finish your entree, don't worry, the fox will eat it," Meyers said.
Now, in the latest issue of "Rolling Stone," Trump, um, lets his hair down, explaining, step by step, how he cares for it.
"OK, what I do is, wash it with Head and Shoulders. I don't dry it, though. I let it dry by itself. It takes about an hour. Then I read papers and things," he said.
"I then comb my hair. Yes, I do use a comb. … Do I comb it forward? No, I don't comb it forward."
According to "Rolling Stone," Trump then pushed "the leading edge of the flying wing of his hair back, to show where the hairline is."
"I actually don't have a bad hairline," Trump said. "When you think about it, it's not bad. I mean, I get a lot of credit for comb-overs. But it's not really a comb-over. It's sort of a little bit forward and back. I've combed it the same way for years. Same thing, every time."
Actually, this is not the first time Trump has opened up about his locks. He devoted a three-page chapter of his 2004 book, "Trump: How to Get Rich," to the topic. Its title? "The Art of the Hair."