The acquired immunodeficiency syndrome is a new disease whose cause is unknown but is almost surely due to a transmissible agent, most likely a virus. The disease is clearly spread by sexual contact, particularly homosexual activity. Blood-borne transmission constitutes the other major recognized form of spread of the disease, although it is highly likely that the disease is not readily spread through casual, nonsexual, non-blood-borne routes. Although the disease is still highly concentrated in the United States, it is now seen in several countries throughout the world. The common denominator of the disease is a profound suppression of cell-mediated immunity, specifically a quantitative and qualitative defect in the T4 inducer or helper subset of T lymphocytes. Hyperactivity of B lymphocytes is also characteristic. The clinical manifestations are those of severe and life-threatening opportunistic infections and unusual neoplasms, particularly Kaposi's sarcoma. The mortality may well approach 100%, making this one of the most extraordinary transmissible diseases in history.