Leonardo DiCaprio Reveals Biggest Regret in Film Career

Leonardo DiCaprio regrets not hooking up with Rollergirl and Amber Waves back in the 1990s ... admitting he wishes he took the lead role in "Boogie Nights."

The actor sat down with Paul Thomas Anderson -- director of the iconic 1997 film about the porn industry, who also directed Leo in the upcoming flick "One Battle After Another" -- for an Esquire interview ... and, he says he regrets turning down the lead role in a "profound movie of my generation."

DiCaprio admits he can't see anyone besides his pal Mark Wahlberg -- who he actually recommended for the role after they appeared in "The Basketball Diaries" together -- in the part, and he calls the movie a masterpiece ... but, he would've liked to be in it.

If you're wondering why he turned it down, Leo was busy with a different role ... a little film you might know called "Titanic" -- which is a pretty iconic movie from DiCaprio's generation too.

DiCaprio and Anderson talked about why it took so long for the two of them to work together ... and LD says he's been trying to work with PTA for two decades -- and "One Battle After Another" was just the right film to make together.