To gain further insight into the possible role of interleukin (IL)-18 in HIV-1 infection we examined serum levels of IL-18 in various clinical and immunological stages of HIV-1 infection during cross-sectional (n = 41) and longitudinal testing (n = 20) and during HAART (n = 21, 24 months follow-up). Our main findings were that HIV-1-infected patients had significantly raised IL-18 levels comparing healthy controls, particularly in those with advanced disease, that while HAART induced a marked decline in IL-18, virological treatment failure was associated with persistently raised IL-18 levels during such therapy and that our in vitro experiments showed an IL-18-mediated up-regulation of the HIV-1 coreceptor CXCR4 and the pro-apoptotic mediator TRAIL in PBMC from HIV-1-infected patients receiving HAART. HIV-1 infection appears to be characterized by persistently raised IL-18 levels and during HAART, such a pattern was associated with virological treatment failure, possibly contributing to immunodeficiency and HIV-1 replication in these patients.