HIV infection predisposes to several neoplastic conditions, especially non-Hodgkin lymphoma (NHL) and Kaposi's sarcoma (KS), and also intraepithelial cervical neoplasia (CIN) and anal neoplasia (AIN) (but not cervical or anal invasive cancer) and possibly seminoma. For neoplasias associated with oncogenic human viruses (ie, some NHL, CIN, AIN, and probably KS) the role of HIV is most probably linked to its immunosuppressive effect and interference with immune-mediated tumour surveillance. HIV-1, through its regulatory protein tat, might also have a direct promoting effect on KS lesions but it is not essential for their development. The increased frequency of Burkitt's lymphoma and Epstein-Barr-virus-negative large-cell lymphoma in AIDS patients, but not in immunosuppressed transplant patients, and the increased rate of testicular tumours in HIV-infected individuals remain unexplained and may indicate either a direct role for HIV or other cofactors.