Circulating immune complexes are part of normal immune defense mechanisms and, therefore, present in various infectious--bacterial and viral--diseases. On the other hand, they are obviously involved in pathogenic mechanisms, e.g., autoimmune diseases or different forms of malignancies. Both autoimmune and infectious features are recorded in the acquired immunodeficiency syndrome. Thus, elevated levels of antigen-antibody complexes in HIV-infected persons had to be expected, and they were in fact demonstrated by several authors. In a cohort study, it was additionally shown that circulating immune complexes are of prognostic relevance. After an introduction concerning the physiological and pathophysiological role of circulating immune complexes in general, their involvement in the course of HIV infections is presented and discussed. In addition, there is a critical review of the most commonly applied assay systems for the detection and quantification of circulating immune complexes.