A 13-year-old boy with meningiomatosis, McCune-Albright syndrome, and gray platelet syndrome presented with an enlarging “lump” on his right forehead. A head CT scan revealed a polyostotic fibrous dysplasia involving the entire skull. A 3.4-cm right frontal osseous cavity and an overlying right forehead subcutaneous soft-tissue mass were seen, measuring 5.2 cm in diameter and 1.6 cm thick. Ultrasound of the cavity and overlying mass showed swirling of blood and an arterialized waveform. MRI revealed an en plaque meningioma underlying the cavity. An intraosseous pseudoaneurysm fed by 3 distal anterior division branches of the right middle meningeal artery (MMA) with contrast extravasation was found on angiography. Two MMA feeders were embolized with Onyx, with anterograde filling of the intraosseous cavity with Onyx. A small pocket of residual intracavity contrast filling postembolization from a smaller third MMA feeder eventually thrombosed and the forehead lump regressed.