Low CD4+ T-cells in the absence of known etiology has been called idiopathic CD4+ T lymphocytopenia, and a case definition was established by the CDC as a CD4 count less than 300 cells/mL on 2 consecutive samples in a patient uninfected by HIV and with no other disease or treatment that might provide a reasonable alternative explanation for CD4 lymphocytopenia. Early investigations centered on a search for a novel retrovirus, but we and others have not found evidence of such a retrovirus. The patients that we have seen are heterogeneous, and it appears likely that there will be multiple etiologies for this syndrome. We are currently continuing to follow subjects long term and to see new subjects in order to continue to characterize the clinical aspects of patients who meet the case definition; to determine the natural history of ICL, and to provide samples to researchers who are interested in investigations into the pathogenesis of ICL.