From Popeater.com:

While no details have been released as to how the smooth singer — real name Nathaniel Hale — died, it has been reported he suffered strokes in both 2007 and 2008.
Nate Dogg was a collaborator with hip-hop megastars including Eminem and Dr. Dre, and he is a founding member of the hip-hop group 213 with Snoop Dogg and Warren G.
Just last week, Warren G tweeted that Nate Dogg was receiving treatment. “For those that dont know awhile back nate had 2 strokes he is in therapy thanks again for your support,” he tweeted.
