During an histological study of pulmonary thromboembolism, arterial wall splits, many associated with saccular microaneurysms, were observed in association with emboli or their fibrous residue. Other aneurysmal lesions, non-inflammatory focal medial necrosis, and medial scars were also seen in a few cases. The nature of the arterial rents indicates that they arose by mechanical splitting after acute stretching of the artery at the time of embolic impaction. Saccular aneurysms then developed in some. Impaction could also have caused the foci of medial necrosis while the medial scars could represent healed former necrosis of rupture. Though the observed incidence of lesions was relatively low, this seems to be due to their small sizes and the sampling inherent in conventional histological analysis. It is concluded that arterial splits are probably a common effect of pulmonary embolism and often give rise to small aneurysms.