'Simpsons' Star Marcia Wallace -- Breast Cancer DID Contribute to Death

"Simpsons" star Marcia Wallace's family denied breast cancer had anything to do with the actress' death ... but according to her death certificate, the disease played a significant role.

TMZ obtained a copy of Wallace's death certificate ... which lists pneumonia, sepsis (inflammation due to infection), and chronic obstructive pulmonary disease (COPD) as the primary causes of death.

The report also says breast cancer was a "significant condition" contributing to her death ... despite the fact Marcia's son initially downplayed reports mentioning cancer.