Tumor treatment and transplantation-associated with unavoidable mandatory immunosuppression-appear to be unreconcilable opposites. The clinical reality shows, however, that transplantation in many early stage primary tumors is the most effective treatment. The essential immunosuppression after transplantation can however promote tumor recurrence. Immunosuppression also leads to a significant increased rate of de novo tumors-in all organ transplant recipients. However, not all immunosuppressant drugs have the same effect on tumors. In experimental and clinical settings, the class of mTOR inhibitors has a clear antitumoral effect and is recommended as the immunosuppression treatment of choice in patients with increased tumor risk. The purpose of this review is to provide the reader with the scientific background regarding the clinical problem of tumors and transplantation.