This report presents a case of a 2-month-old female German Shepherd dog with renal nephroblastoma and strong cachexia. The tumour was surgically removed and graded as a stage I with favourable histopathology, but due to its large size, adjuvant chemotherapy, including doxorubicin and vincristine, was recommended. Before, during and after chemotherapy, the body weight was gradually rebuilt with an appropriately balanced diet containing proper amounts of protein, fat and numerous supplements necessary for growth and development. Eighteen months after diagnosis, the dog was in good condition and had no metastases. In conclusion, this case study shows that close cooperation between oncologists and nutritionists is crucial for successful oncological therapy, especially in a cachectic patient.