Solid-organ transplant recipients are at a 3- to 5-fold increased risk of a de novo malignant neoplasm developing compared with the general population. The most frequently developed virus-associated malignant neoplasms are Kaposi sarcoma (standardized incidence ratio [SIR], 208.0), nonmelanoma skin cancer (SIR, 28.6), and posttransplant lymphoproliferative disorder, primarily non-Hodgkin lymphoma (SIR, 8.1). Immunosuppressive agents such as corticosteroids, antimetabolites, calcineurin inhibitors, and mammalian target of rapamycin (mTOR) inhibitors play a key role in either causing or preventing this complication. It is hypothesized that some of these regimens can impair cancer surveillance, facilitate the action of oncogenic viruses, and promote direct oncogenic activity. Evolving research has shown promising dual antitumor and immunosuppressive properties of the mTOR inhibitor class. The effective management of posttransplant neoplasms most likely involves the use of these medications among other preventative options. These measures include monitoring certain viral loads as well as immunosuppressant drug levels. Reducing these levels to as low as possible for healthy engraftment and altering regimens when appropriate are management strategies that could lessen this complication of solid-organ transplant. More studies examining the effects of therapeutic drug monitoring are needed to determine specific plasma drug concentrations that will ensure organ engraftment without the development of de novo malignant neoplasms.