The study comprises 49 dogs, 2 horses, and 2 cows with haemangioendothelioma, which have been subjected to autopsy over a period of 26 years. Most of the dogs were old, and there is an over-representation of male dogs. The results also indicate that the Alsation breed of dog is particularly susceptible to these tumours, as is the Boxer to some extent. The disease was in most cases of short duration and the most common clinical symptoms were sluggishness, fever, generalized anaemia, breathing difficulties, increased thirst, and leuco-cytosis. Most of the dogs fell ill and were subjected to autopsy during the period January-May. The lungs and the heart were the most frequently affected organs, but other sites were also represented, and in most cases the tumours were multiple. The right atrium, and notably the auricle, appeared to be particularly susceptible to this type of tumour, which, owing to its haematogenic spread, was mostly found in highly vascular organs. Ruptures of the tumours were common findings, together with resulting haemorrhages to the abdominal and thoracic cavities and to the pericardium. Other tumour types, mostly benign, were seen to co-exist, and among other common autopsy findings were jaundice and transudations into serous cavities. The histological examinations indicate that the tumours can be both infiltrative and circumscribed, and that varying degrees of differentiation can be found even within the same tumour. Hyalinization and ossification of the tumour stroma occur, and the presence of haemosiderin and phagocytic cells is common. As regard the 2 horses and the 2 cows described separately, it was found that the organs involved in these cases were the same as those that prove to be most commonly involved in the dog. In other respects, too, the findings in these cases seem to agree well with those made in dogs.