Stan Lee passed away from heart failure.
The comic book legend - who died on November 12 - passed away from heart failure and breathing problems, according to his death certificate.
TMZ reports that Stan was also suffering from aspiration pneumonia, which happens as a result from inhaling food, stomach acid or saliva into the lungs.
Stan - who was 95 when he passed away - was laid to rest after a "private closed ceremony" on Friday November 16.
Stan's POW! Entertainment company shared a message on Twitter, which read: "Stan was always adamant that he did not want a large public funeral, and as such his family has conducted a private closed ceremony in accordance with his final wishes.
"Our thoughts and prayers continue to be with him. We at Stan Lee's POW! Entertainment are working on putting together a tribute befitting the greatest creator of our time and the father of modern pop culture.
"The grandeur of Stan makes this a monumental task, and we hope to have more info in the days to come. In the meantime, we understand and share your desire to commune with fellow fans, and we have set up a tribute wall on therealstanlee.com where we can all share our thoughts, prayers and messages of support. We look forward to joining all of Stan's fans in celebrating his extraordinary life and legacy."
Stan began his career as a comic-book writer, editor, film executive producer, actor, and publisher, in 1935, and has created or co-created some of the most famous comic book characters - and later on-screen heroes - that are still loved by millions today.
In collaboration with artist Jack Kirby, Stan created the likes of Black Panther, Nick Fury, the Hulk, the X-Men, the Fantastic Four, the Inhumans, and Scarlet Witch, as well as teaming up with his own brother Larry Leiber and Jack on the creation of Ant-Man and Thor, and again with Jack, Larry, and Don Heck on one of Marvel's most iconic superheroes, Iron Man.