Two cases of calvarium sarcoma with intracranial extension, consecutive to Paget disease of the skull are presented. The neurosurgical implications of this rare complication of a common disease are reviewed. In both cases, the symptoms of malignant degeneration were unspecific and blurred by the presence of a well-known Paget disease. Increasing headaches, focal neurological deficits and signs of intracranial hypertension were observed. Skull X-rays and Computerised Tomography did not allow to make the difference between Pagetic bone and sarcoma. In the second case, Magnetic Resonance Imaging was the most accurate in determining the precise localisation of the tumour, and the subdural invasion. Extensive surgical resection was carried out, by craniectomy of the tumoural bone, followed by cranioplasty. Dural sinus involvement and tumour hypervascularisation caused important peroperative blood loss in both patients. In the second case, the tumour invaded the subdural space through a Pacchioni granulation. Survival of patients with Pagetic sarcoma seems shorter when compared to post-radiation or primitive osteosarcoma, despite adjuvant therapies, probably because of late diagnosis and incomplete surgical resection. The mean reported survival rate is 6 months. Although the first patient died within 4 months, in the second case, the authors obtained a more than two year survival with aggressive surgery alone. These cases also illustrate the polymorphism of sarcomatous degeneration of Pagetic calvarium, the interest of MRI, and the need for close surveillance of patients with known Pagets disease of the skull.