The classic 1950sHarvey cartoonsalways seemed to gloss over the subject. However Casper's cause of death was finally offered up in the 1995big screen adaptionstarring Christina Ricci. Apparently the friendly ghost was originally a stubborn kid. Casper was sledding on a cold winter's day. His dad told him to come inside. Casper refused, and eventually contracted a fatal case of pneumonia. The end.