With the discovery of human immunodeficiency virus (HIV), researchers identified the etiologic agent for acquired immunodeficiency syndrome (AIDS). Although neither a cure nor a vaccine exists for this fatal disease, considerable information is now available on the progression from HIV infection through AIDS. Understanding the natural history of this progression can facilitate early detection and intervention. Although the epidemic continues to claim lives at an accelerating rate, the timely administration of prophylactic and therapeutic modalities can significantly prolong survival time for many patients. As more effective and less toxic treatments are developed, it will become increasingly crucial for practitioners to accurately diagnose and stage HIV-infected patients.