Sylvester Stallone’s eldest son, Sage Stallone, died in 2012 at age 36. Reports later said the cause was heart disease, and his death was a major loss for Stallone and his family.

What happened

Sage was found dead in his Los Angeles home on July 13, 2012, after he had not been heard from for several days. Early reports speculated about an overdose, but the later coroner findings pointed to coronary heart disease caused by atherosclerosis, with no illicit drugs found.

Who Sage was

Sage appeared with his father in Rocky V and later worked in film as an actor and director. In later years, Stallone spoke publicly about the grief of losing him and reflected on how deeply it affected the family.

Another son

Sylvester Stallone also has a second son, Seargeoh Stallone, who has lived mostly out of the spotlight and was diagnosed with autism at a young age.

TL;DR: His son Sage died in 2012, and the later official explanation was heart disease, not foul play.