The article reports on a case of an aneurysmatic bone cyst which deteriorated into a malignant cyst four years after the first exposure to roentgen radiation, developing massive metastases in the lung after two further years. Almost all malignant courses of an aneurysmatic bone cyst which have been described in literature (a total of 12 cases so far) developed subsequent to x-ray treatment. The case reported here is unusual in so far as the malignant transformation took place after a short while only, whereas the total radiation dose was relatively low. The authors sound an urgent warning against x-ray treatment of aneurysmatic bone cysts.