The occurrence of multiple malignant tumours has been recorded, usually as individual cases but also in groups from tumour registries or autopsy studies. A personal series of 71 cases encountered in the follow-up clinics of two District General Hospitals is presented. Cases of multiple skin tumours, and of intestinal growths associated with pernicious anaemia, ulcerative colitis and familial polyposis have been excluded. Review of the literature has been carried out and possible aetiological factors have been considered. The importance of the functions of the tumour surveillance mechanism is emphasized, and the possibility of depression of the defence mechanism by major surgery, is considered to be of importance. Assessment of the patient's defence mechanisms may be as important as grading and staging the tumours.