Acquired immunodeficiency syndrome (AIDS)-related lymphomas consistently display a B-cell phenotype and are histogenetically related to germinal centre or post-germinal centre B cells in the overwhelming majority of cases. The pathogenesis of AIDS-related lymphoma is a multistep process involving factors provided by the host as well as alterations intrinsic to the tumour clone. The molecular pathways of viral infection and lesions of cancer-related genes associated with AIDS-related lymphomas vary substantially in different clinicopathological categories of the disease and highlight the marked degree of biological heterogeneity of these lymphomas.