Majority of the Western population is infected with the Epstein-Barr virus. Except for mononucleosis accumulating in adolescents, the infection occurs in a rather symptomless manner. But later on under special circumstances the virus may play an important role, again. As a result of long-lasting and intensive immunosuppression i.e. following transplantation a lymphoproliferation of B cell origin may develop in some patients. The biologic behaviour is often more aggressive than expected by the histological occurrence. Early identification of predisposed patients is an important but difficult task: the disease exhibits a rather heterogeneous phenotype and the grading is problematic by the standard classification criteria of non-Hodgkin malignant lymphomas. Nevertheless the diagnosis and the therapy based on the appropriate pathomechanism can increase the chances of convalescence. This review article discusses the pathomechanism, diagnostic and therapeutic relevance of the posttransplant lymphoproliferative disease.