It is well known that acute alveolar haemorrhage (AAH) is attributed to capillaritis in most cases with microscopic polyangiitis (MPA). In this article, we explore the cause of alveolar haemorrhage in MPA patients. In the present study, we extracted four autopsy cases of MPA with AAH. Patient’s sex and age, cause of alveolar haemorrhage, therapy, follow-up duration, and cause of death were investigated. As a result, alveolar haemorrhage was caused by diffuse alveolar damage (DAD) due to candidiasis or influenza virus infection, haemorrhagic infarct due to aspergillosis, capillaritis due to MPA, vasculitis due to cytomegalovirus (CMV), and herpes simplex virus (HSV) infection. All patients received corticosteroid therapy, and one patient additionally underwent administration of cyclophosphamide. The duration of follow-up ranged from one to 26 months with a mean of eight months. All patients died of respiratory failure. In summary, clinicians and pathologists should recognise some causes of alveolar haemorrhage in MPA patients, which include DAD, haemorrhagic infarct, virus-associated vasculitis, or MPA-associated capillaritis.