CHRONIC pyelonephritis is a generic term meaning different things to different people. To perhaps the majority it signifies the small, coarsely scarred kidney of what is also known as "atrophic pyelonephritis", which gives rise to hypertension in young people, or, if it is bilateral to renal failure and death. To some it embodies a histological picture which may be found in many conditions and which it is not always possible to associate closely with infection. To the clinician it means recurrent or relapsing urinary infection, and it is notoriously lacking in clearly defined clinical features in many instances. To the bacteriologist it is a state in which abnormal numbers of bacteria appear, or may be caused to appear, in the urine from time to time. There is in fact no single entity which includes all these requirements, all the time. Nor indeed were it reasonable to expect one in so complicated an organ, any more than the term 'chronic lung infection' might be expected to refer to suppurative pneumonitis rather than cavitating tuberculosis. Radiology's contribution to this subject is that it can define a coarse renal scar by excretion pyelography, and it enables both Wan early diagnosis to be made and the natural history of the disease to be studied. The sign of a scar is a localised loss in thickness of the renal substance. There is usually disappearance of the adjacent pyramid also. (Fig. 1). The definition of this lesion, therefore, depends on the demonstration, during pyelography, of both the renal outlines and the calycine details, which in turn depends on care with radiographic technique and the preparation of the patient's abdomen before examination. We have shown that this is a perfectly possible everyday matter in a busy X-ray department. (Fig. 2). Such scarring is essentially irregular in its distribution, albeit it tends to occur in certain patterns. It is therefore to be distinguished from the uniform change seen equally throughe........