The acquired immune deficiency syndrome (AIDS) was recognized as a distinct entity in 1981. It began as a medical curiosity affecting only several dozen individuals in a restricted segment of the U.S. population. In the 12 years since its description, AIDS has become a pandemic affecting tens of millions with cases reported from all major countries. The illness is caused by a retrovirus, termed human immunodeficiency virus (HIV). It is a blood-borne disease with sexual, parenteral, and perinatal modes of transmission. Infection with the virus can be determined by a number of serologic techniques as well as viral culture. The pathophysiology of illness is incompletely understood, but is in large part related to destruction of helper, CD4 lymphocytes. This results in immune dysfunction and the development of a variety of opportunistic infections and malignancies. A great deal has been learned over the last decade, with important advances in treatment. Zidovudine (AZT) remains the most important agent in slowing progression of the disease and has resulted in prolonging survival. All organ systems can be affected by HIV, and many clinical manifestations are protein. Fever, weight loss, and diarrhea are often encountered general symptoms. The skin is frequently involved, with Kaposi's Sarcoma the most common malignancy and a variety of fungi and viruses the most frequent cause of infection. The lung is involved in the majority of patients, with Pneumocystis Carinii (PCP) and mycobacteria emerging as the most important pathogens. A variety of treatments have demonstrated efficacy for PCP. The risk of PCP is related to the decay in CD4 lymphocytes so that prophylactic treatment is recommended when CD4 counts fall below 200. Mycobacterial infection with multiresistant organisms has complicated the management of these infections and poses new risks to health care workers. Part 1 of this two-part series on AIDS discusses the pathophysiology and clinical expression, epidemiology, laboratory testing, and the general clinical manifestations of AIDS, as well as dermatologic, pulmonary, and cardiac symptoms. Part 2 will discuss the gastrointestinal, neurologic, and ocular symptoms, as well as the treatment and management of the AIDS patient.