The most frequent renal tumours of animals are renal cell carcinoma and nephroblastoma. Renal cell carcinomas are seen mainly in dogs and cattle and nephroblastoma is encountered in pigs, puppies, and calves. Renal cell carcinomas are usually papillary in the dog. They show a marked propensity for vascular invasion, penetration of the posterior vena cava, and subsequent pulmonary metastasis. Nephroblastoma, which is morphologically identical to Wilms' tumour of children, is almost always a benign tumour in animals. It is one of the most frequent neoplasms of pigs, possibly owing to the fact that most pigs are slaughtered (and examined) when a few months old. Lymphosarcoma involving the kidney is particularly frequent in the cat, but is also seen in other species as part of a generalized disease.