An adrenal cyst in a 36-year-old apparently healthy woman was diagnosed as a hamartoma with haemangiomatous structures and a large cyst, probably secondary to haemorrhage within the lesion. After reviewing the histological records of all the previously published cases of similar "haemorrhagic" non-inflammatory adrenal cysts, the aetiology of which has been considered unclear, we conclude that they too were probably hamartomas. We therefore propose that this type of adrenal cyst, the only of clinical importance, is aetiologically a hamartoma. These tumours often show super-imposed degenerative post-haemorrhagic changes.