Opportunistic disseminated strongyloidiasis is an important cause of morbidity and mortality in immunocompromised patients. However, the worldwide prevalence of Strongyloides stercoralis is difficult to determine because adequate data are lacking. In this paper more than 100 epidemiologic studies reporting on the prevalence of S. stercoralis among various populations on five continents are critically reviewed. Analysis of this information indicates that the following groups of people may be at risk: residents of an emigrants from any developing country and southern, eastern, and central Europe; travelers and veterans returning from endemic areas; natives and residents of the Appalachian region in the United States and local endemic areas in other countries; and institutionalized persons. Because disseminated infections may be prevented by early treatment of asymptomatic chronic infections, screening programs are recommended to detect latent S. stercoralis infection before the initiation of chemotherapy or immunosuppression in patients at risk. Serologic tests are sensitive and specific but are not yet widely available. Thus strong suspicion based on clinical and epidemiologic clues in conjunction with repeated stool examinations remains the best way to rule out S. stercoralis infection in these groups.