Within the context of HIV disease, a marker may be described as a consequence of disease that varies over time but does not necessarily predict future disease course. To date, the most powerful marker of HIV disease progression is the CD4 cell count. Other immunologic markers include neopterin, beta 2-microglobulin, and total and HIV-specific immunoglobulin levels. Further research, which focuses on cell-mediated factors such as interleukins, tumour necrosis factor, natural killer cell activity and apoptosis, is required. Measures of viral burden, such as p24 antigenemia and proviral DNA or RNA, may also offer additional prognostic information. As methods involving quantitative polymerase chain reaction become more refined, it is hoped that they may soon be applied to the clinical setting. Clinical markers of interest include the appearance of minor opportunistic infections and the occurrence of acute retroviral syndrome, which may indicate a faster disease course. Although population-based studies have identified a number of HIV disease markers, further research is required to generalize these findings to the individual level.