It has been said that "dog is man's best friend'. Comparative oncologists are demonstrating that pet dogs could hold the keys for advances in cancer treatment in people. Specific types of cancer that arise spontaneously in pet dogs could serve as important models of human cancer, having much greater similarity to their counterparts in humans than many currently used experimentally induced tumor models. In vivo models that are relevant to human cancer are greatly needed, particularly for evaluating new strategies for cancer therapy, such as augmentation of the immune system or blockade of the metastatic cascade. In addition, pet dogs could also be used to identify environmental carcinogens by acting as sentinels of cancer induced by these toxins.