[WITH SPECIAL PLATE] Osteomyelitis of the base of the skull appearing together with or soon after infection of the sinuses or middle ear presents no diagnostic difficulties. This is not the case, however, where chronic osteomyelitis develops without such previous manifestations. Differentiation from neoplastic disease, in particular, is then by no means easy, nor is it clear whether the inflammation is primary or has developed by a process of extension from neighbouring tissues. Radiographs of the skull, even when abnormal, do not give unequivocal information as to the nature of the lesion. Signs The symptoms and signs as described above were thought to be consistent with a diagnosis of cerebellopontine-angle tumour, though the minimal hearing disturbance, and in particular the site of the bony defect as shown by x-rays, were not appropriate for an acoustic neurinoma.Suboccipital craniotomy was performed, and when the left cerebellar lobe was raised the dura mater in the region BITrIsH MEDICAL JOURNAL