Non-Hodgkin's lymphoma consists of a group of disorders which is increasing for the last decades. In the USA, since the early 1970s, the incidence rate has increased by almost 60% and ranges from 3 to 4% per year, which is faster than for the majority of cancers. Important advances in our knowledge of histopathology together with new diagnostic tools, as well as the improvement of the registered data and ageing of the population, only partially explain this increase. Two viruses have been identified as possible etiologic agents for non-Hodgkin's lymphoma: the Epstein-Barr virus and HTLV-I. More recently, HHV8 and the hepatitis C virus have been incriminated in the occurrence of non-Hodgkin's lymphomas. Congenital and acquired immunodeficiencies (due to immunosuppressive therapy and viral disease such as Aids) might also represent predisposing factors. However, the epidemic HIV infection is the most strongly correlated with the increasing incidence of non-Hodgkin's lymphoma in the United States. Finally, epidemiologic studies indicate that environmental factors may play an important role in the etiology of non-Hodgkin's lymphoma, specially toxic chemicals used in farming.