Human herpesvirus 8 (HHV-8), a lymphotropic gammaherpesvirus, is the etiologic agent of Kaposi's sarcoma (KS). Results from serologic assays for HHV-8 infection have variable concordance, primarily in low risk populations. The detection of HHV-8 infection could be improved by optimization of existing serologic tests, development of new assays and effective algorithms, better understanding of the immune response, and identification of new diagnostic antigens. To address this, a number of HHV-8 serologic tests were developed or modified and compared to existing methods. IgG, IgK and IgA isotype seroprevalences against the Orf 65 antigen were 93%, 87%, and 40% in KS samples, respectively---much higher than observed in USA blood donors (BDs: <3%). In comparison studies, the K8.1 IgG ELISA had the best overall diagnostic performance. For seropositive BDs, there was poor agreement among multiple tests. HHV-8 seroprevalence in non-KS samples was highest in Africa (9%--42%), followed by the Middle East (6%--20%), Asia and South America (0%--18%), the USA (0%--11%), and the Caribbean (1%--2%). Rates were highest in KS patients (74%--100%), high in African BDs (22%), lowest in non-African BDs (2%--7%), and significantly higher in HIV+ individuals (17%) as compared with HIV- (8%). Antibody reactivities using multiple assays were observed in patients with sarcoidosis (0%--33%: P = 0.007) and multiple myeloma (0%--21%: P = 0.017). A diagnostic algorithm was established exhibiting high sensitivity (99%) for KS samples using an Orf 65 ELISA followed by a lytic IgG IFA. A study of the kinetic humoral response to HHV-8 showed 4-fold changes in Orf 65 reactivity, and increasing IgM or IgA responses suggested viral reactivation. In addition, Western blot analysis showed temporal IgG and IgM reactivities against specific HHV-8 lytic proteins. Epitope expression proved effective in identifying a new HHV-8 antigen; antibodies to the Orf 64 tegument protein had a higher seroprevalence in KS sera (30%) than in BDs (2%). In summary, HHV-8 diagnostic tests were developed or modified to have improved test indices, prevalence studies showed the utility of the tests, antibody isotype reactivity against HHV-8 was better characterized, the kinetics of the immune response were better defined, and a new HHV-8 antigen was discovered through epitope expression.