The diagnosis of most cholesteatomas of the middle ear is made by direct examination of a perforation in the eardrum, and the only radiological investigation necessary for these patients is a set of plain petro-mastoid views. Of these the lateral with caudal tilt of the tube is the most useful, since it demonstrates the extent of pneumatisation and the position of the lateral sinus and middle fossa dura. The radiographic appearances do not generally affect the surgical approach or the course of the operation which must be exploratory. Nevertheless multidirectional or computerised tomography can demonstrate the soft tissue mass of a small cholesteatoma as well as the typical attico-antral erosion and ossicular displacement and destruction that occur. Tomographic investigation is needed when the diagnosis is in doubt, as when the eardrum is obscured, or when complications develop, but should not cause delay in instituting necessary surgery. For the rare congenital cholesteatoma in or close to the petrous pyramid with an intact eardrum, tomographic studies are essential to show the extent of the bone erosion, and to plan the surgical approach. The appearance of a smoothly outlined expansile lesion in the petrous pyramid may be almost pathognomonic, especially if this is shown to have low attenuation values after contrast enhancement. The role of high resolution CT in these cases is discussed.