Before scrubbing up for his Hollywood fame, Brad Pitt would happily go days without showering -- at least according to Jason Priestley ... who used to live with the dude.

The '90210' alum reflected on their brief time living together pre-fame along with one other person in a "crappy part of L.A." ... telling the tale on "Live with Kelly and Mark" Tuesday.

JP says the roomies would frequently play a game to see who among them could go the longest without bathing. And as it turns out, Jason says there was a consistent winner.

According to him, it was Brad who usually made a clean sweep of the disgusting competition ... which leaves us wondering if he still would've become Hollywood's biggest heartthrob if people caught wind of this alleged stink-fest back then.

Thankfully, Jason says he doesn't think he's into those games anymore, given his flashy A-list status and all -- although he says he can't confirm this definitively.

Luckily for both J&B, their days cramped up together in a small apartment didn't last long as they both catapulted to fame in the early nineties -- Jason in "Beverly Hills, 90210" & Brad in "Thelma and Louise." After that, the rest was history.

Safe to say, they're anything but washed-up actors these days ... although, it sounds like BP might've needed a bath or two while he was trying to break into the biz.

Fortunately, those days are behind them, and their careers have been showered with praise.