This paper reviews the impact of veterinary cancer epidemiology on veterinary oncology, human oncology, comparative oncology, and on the etiology and pathogenesis of cancer. The detection of clusters of diseased animals has led to the discovery of the infectious, viral-associated nature of malignant lymphoma of cats, poultry, and cattle. Although some viruses (FeLV, BLV) can, under experimental conditions, cross the species barrier, there is thus far no evidence for a zoonotic hazard for the human. The keeping of pet/birds or pigeons was found to be associated with an increased risk of lung cancer in the bird keepers. Dogs appear to be useful 'sentinels' for environmental hazards (asbestos, dyes, passive smoking, insecticides). The complex pathogenesis of cancer was dissected in an epidemiologic-experimental study in cows, which had intestinal papillomas and carcinomas. Endogenous genetic factors may also play a role in pathogenesis, as is evidenced by species, breed (Boxer!), and family related aggregates of tumour diseases. Epidemiology may provide a means to prevent tumour diseases by, for example, withdrawal of hormones (mammary cancer) or isolation of tumour-virus positive animals (malignant lymphoma).