Andrew Breitbart -- founder of the popular news website Breitbart.com -- died early this morning at the age of 43.

According to his website, Breitbart -- the political journalist who broke the Anthony Weiner penis pic story -- died "unexpectedly from natural causes" shortly after midnight this morning in L.A.

A statement on Breitbart reads, "We have lost a husband, a father, a son, a brother, a dear friend, a patriot and a happy warrior."

Andrew is survived by his wife and 4 children.

Breitbart's father-in-law told reporters Andrew was walking near his Brentwood home around midnight when he collapsed.

The father-in-law says someone saw Andrew collapse and called paramedics ... who raced to the scene and tried to revive him. Breitbart was transported to UCLA Medical Center where he was pronounced dead.

According to reports, Breitbart suffered from heart problems last year.

Law enforcement sources tell us the LAPD is investigating the death.