The United States pet population has been a vastly underutilized resource for cancer therapy studies. Naturally occurring tumors in dogs develop twice as frequently as in man, have histopathologic features and a biologic behavior similar to tumors in man, and progress at a more rapid rate than in man. Cancer is one of the leading causes of death in man and dogs. The canine malignancies that are of practical use for comparative therapeutic studies include lymphoma, mammary tumors, oral melanoma, lung tumors, nasal tumors, soft tissue sarcomas, and osteosarcoma. This report will discuss the comparative nature of these malignancies and the current trends in clinical cancer research, namely dose intensification and biomodulation, using naturally occurring tumors in client-owned dogs.