Death represents the main hallmark of amyotrophic lateral sclerosis (ALS). Despite its importance in clinical care and phase III trials, many uncertainties remain on the cause of death due to the lack of post-mortem verifications. To provide a more robust approach to these causes, we performed a retrospective pathological study on a large cohort of patients. 100 ALS patients referred for a deterioration of their clinical condition and who died in the ALS clinic of Salpétrière had a complete macroscopic and microscopic post-mortem analysis. The clinical causes of death reported on medical records were compared to the results of autopsy. The concordance between clinical and pathological conclusions was insufficient (20%) to consider clinical assessment as a reliable marker of causes of death. At autopsy, broncho-pneumonia and pneumonia were the main causes of death. Heart failure, representing 10% of deaths, was two times more frequent in bulbar than in spinal ALS. Pulmonary embolism representing 6% of death was exclusively found in spinal onset patients and is related to lower limbs disability. An effort has to be made for a better understanding of the causes of deterioration of ALS patients. A more proactive attitude to treat respiratory infections could have a significant impact on survival.