The aetiology of non-Hodgkin's lymphomas remains a controversial matter, but, recently, evidence has emerged showing that these neoplastic aberrations of the immune system may be due to viruses, at least in some cases. In fact, patients affected by an inherited immune deficiency, and those presenting disease characterized by autoimmune dysfunctions, show an increased risk for the development of non-Hodgkin's lymphomas. Several viruses have been identified as potential aetiologic agents for of non-Hodgkin's lymphomas: one of these is the Epstein-Barr virus, which has been detected in cultures of tumour cells from patients with Burkitt's lymphoma: this virus seems to be involved also in the pathogenesis of some histological variants of Hodgkin's disease. In addition, the human T-cell lymphotrophic virus family members have also been recognized as possible aetiologic agents for several lymphomas, such as cutaneous T-cell lymphomas, T-cell leukaemia and T-cell hairy cell leukaemia. Recently, hepatitis C virus has been recognized as the aetiologic agent of mixed cryoglobulinaemia, which can be considered as a benign lymphoproliferative disorder. Since mixed cryoglobulinaemia can frequently evolve into more aggressive haematological disorders, an increased prevalence of hepatitis C virus infection in non-Hodgkin's lymphomas has been found, especially in low-grade non-Hodgkin's lymphomas. The possible aetiopathogenetic role of hepatitis C virus in non-Hodgkin's lymphomas is discussed on the basis of molecular, clinical and epidemiological considerations.