There was no autopsy, and the cause of Lambert's death is unknown. While many sources say that he died of a fatty degeneration of the heart or of stress on his heart caused by his bulk, his behaviour in the period leading to his death does not match that of someone suffering from cardiac insufficiency; witnesses agree that on the morning of his death he appeared well, before he became short of breath and collapsed. Bondeson (2006) speculates that the most consistent explanation of his death, given his symptoms and medical history, is that he had a sudden pulmonary embolism.