Radiotherapy as the sole method of treatment or as a preoperative measure is an important tool against the inoperable tumours of the kidney in the adult. Several techniques of irradiation are possible, for all of which the CT is an invaluable means of efficiency. The author describes the case of a huge renal tumour in a patient aged 76, exclusively treated by cobalt therapy, who is alive and well at more than 3 years. Four main conclusions are drawn: 1) the treatment must be versatile and continuously adapted to the clinical, radiological and CT modifications; 2) in the first phase of treatment a large tumour must be irradiated through a grid (gamma or high energy X-rays); 3) the fractionation must be very slow (2 sessions a week or several cycles in an overall time of some months, with periods of rest); in order to getting a good re-oxygenation of the large anoxic parts of the tumour mass; 4) the involution of the tumour must be graphically evaluated, as the rate of shrinkage may be very slow, continuing many months after completion of the treatment.