A radiological classification of diseases must cope with a number of exacting demands which make conventional clinical classifications impractical. The present system of classification is designed to meet these demands and facilitate indexing of teaching and research material. It also serves for immediate communication of the radiological interpretation, thus obviating repetitious and often unnecessarily conflicting review of films. The classification remains brief and yet comprehensive by separating anatomical structures, aetiological agents and radiological forms, the components of all disease entities. Each component may be specified by a readily memorised letter and number. Appropriate components are combined to compose each specific diagnosis.Cases may be coded in greater or lesser detail according to departmental policy or the requirements of the individual radiologist. In the majority of cases anatomy and aetiology suffice to define a disease. Thus a colon polyp is A8g1 and pneumonia is R5b1. Many special radiological features may be coded in addition to the disease itself. The code may be recorded on the film envelope for immediate communication purposes or on index cards for permanent record and future recall.The classification fulfils a practical need that has long been felt by academically inclined radiologists. It should lead to more effective recall of valuable clinical material which might otherwise be lost. It should also facilitate the efficient day-to-day operation of the Department of Radiology.