Organ transplantation is an increasingly used medical procedure for treating otherwise fatal end stage organ diseases with 107,000 transplants performed worldwide in 2010. Newly developed anti-rejection drugs greatly helped to prolong long-term survival of both the individual and the transplanted organ, and they facilitate the diffusion of organ transplantation. Presently, 5-year patient survival rates are around 90% after kidney transplant and 70% after liver transplant. However, the prolonged chronic use of immunosuppressive drugs is well known to increase the risks of opportunistic diseases, particularly infections and virus-related malignancies. Although transplant recipients experience a nearly 2-fold elevated risk for all types of de-novo cancers, persistent infections with oncogenic viruses - such as Kaposi sarcoma herpes virus, high-risk human papillomaviruses, and Epstein-Barr virus - are associated with up to 100-fold increased cancer risks. This review, focusing on kidney and liver transplants, highlights updated evidences linking iatrogenic immunosuppression, persistent infections with oncogenic viruses and cancer risk. The implicit capacity of oncogenic viruses to immortalise infected cells by disrupting the cell-cycle control can lead, in a setting of induced lowered immune surveillance, to tumorigenesis and this ability is thought to closely correlate with cumulative exposure to immunosuppressive drugs. Mechanisms underlying the relationship between viral infections, immunosuppressive drugs and the risk of skin cancers, post-transplant lymphoproliferative disorders, Kaposi sarcoma, cervical and other ano-genital cancers are reviewed in details.