The acquired immunodeficiency syndrome (AIDS) is a devastating new disease caused by the human immunodeficiency virus (HIV). This retrovirus causes profound immunoincompetence in its infected hosts, who are thereafter susceptible to develop myriad severe and relapsing protozoal, fungal, bacterial, viral, and arthropodal opportunistic infections, as well as unusual malignancies. The more than 50,000 patients who have developed AIDS in the United States have produced a sudden unexpected deluge of diagnostic dilemmas that are stressing laboratories of pathology everywhere. This paper describes the gross and microscopic pathology of the numerous complications in patients infected by HIV: (a) the prodromal AIDS-related complex with persistent generalized lymphadenopathy, (b) lymphoid infiltration of salivary gland and lung, including the complex of lymphoid interstitial pneumonitis-pulmonary lymphoid hyperplasia, (c) extranodal non-Hodgkin's lymphomas, (d) multifocal mucocutaneous and visceral Kaposi's sarcoma, (e) small cell undifferentiated (oat cell) carcinomas, (f) protozoal infections caused by Pneumocystis carinii, Toxoplasma gondii, Acanthamoeba, Cryptosporidium species (sp.), and Isospora belli, (g) the causes of chronic enteritis, (h) mycotic infections caused by Candida sp., Cryptococcus neoformans, Histoplasma capsulatum, Coccidioides immitis, and Sporothrix schenckii, (i) bacterial infections caused by Mycobacterium avium-intracellulare, M. tuberculosis, M. kansasii, Nocardia sp., Listeria monocytogenes, Legionella sp., Treponema pallidum, and others, (j) viral infections caused by cytomegalovirus, herpes simplex and zoster, polyomavirus (progressive multifocal leukoencephalopathy), hepatitis B, molluscum contagiosum, and papillomavirus, (k) oral hairy leukoplakia, (l) subacute encephalopathy, and (m) Norwegian scabies.