Diagnostic anatomic pathologists play a crucial role in the battle against acquired immunodeficiency syndrome (AIDS). Not only are they intimately involved in the treatment of individual patients with human immunodeficiency virus (HIV) infection, but also they make important observations that result in the expansion of the scientific understanding of its pathogenesis. Pathologists studying tissue from patients with HIV infection should be familiar with the conditions to which these patients are susceptible. Although opportunistic infections are important causes of morbidity and mortality, noninfectious conditions frequently make substantial contributions to the disease course. Patients with HIV infection may be at increased risk for neoplastic disease. They do not, however, have an increased incidence of the most common tumors affecting the general population, such as breast, colon, and prostate carcinoma. Immunodeficiency results in increased susceptibility to malignant neoplasms, both by decreased immunologic response to abnormal cells and increased susceptibility to infection by viruses. All of the malignant neoplastic diseases that are Centers for Disease Control and Prevention (CDC) AIDS indicator conditions have been shown to have an association with a virus: Kaposi sarcoma (KS) with herpes hominis virus 8 (HHV-8), malignant lymphoma with Epstein-Barr virus (EBV), and cervical carcinoma with human papilloma virus (HPV). Patients with HIV infection also can develop reactive processes that are attributable to direct effects of HIV or immune system alterations. Such conditions include salivary gland cystic lymphoepithelial lesion, lymphadenopathy, lymphocytic interstitial pneumonitis, encephalopathy, enteropathy, nephropathy, hepatic conditions, dermatologic conditions and anemia.