The acquired immunodeficiency syndrome continues to be a major public health problem in the United States, and recently its spread worldwide has accelerated. The syndrome is caused by a human retrovirus transmitted by sexual contact and via blood or blood products. The virus has been isolated, characterized, and cloned, and in addition to its presence in blood, it has been found in body tissues and fluids including brain, semen, and saliva. Although the syndrome in the United States is still largely confined to male homosexuals and intravenous drug users, there is increasing evidence, particularly from Zaire, that the virus can be spread by heterosexual contact. Attempts at immune reconstitution with lymphocytes and lymphokines have resulted in some transient improvement in immune function but without clinical effect, indicating the need for specific anti-retroviral therapy in combination with immune reconstitution.