Over the last 10 years the appreciation of the full breadth of the spectrum of human immunodeficiency virus (HIV) infection has gradually increased; it is now known that AIDS represents merely the end stage of this progressive infectious process. The surrogate marker that most closely correlates with the stage of HIV infection is the CD4+, or T helper, cell count. Because this count is relied on in making important therapeutic decisions, it is of paramount importance that clinicians be cognizant of and fully understand the multiple factors that can influence this parameter: the variability of the count from day to day and from morning to night, the influence of intercurrent viral infections, the influence of drugs. In this AIDS Commentary, Sten H. Vermund and his colleagues at the National Institutes of Health discuss these issues and put the CD4+ cell count into a lucid and practical clinical perspective.