To determine the etiology of bloodstream infections (BSIs) in hospitalized patients >/=15 years old in Thailand, prospectively enrolled, consecutive febrile (>/=38 degrees C) patients were admitted to one hospital during February-April 1997. After a patient history was taken and a physical examination was performed, blood was obtained for comprehensive culture and human immunodeficiency virus (HIV) testing. Of 246 study patients, 119 (48%) had BSIs, and 182 (74%) were infected with HIV. The 2 most common pathogens were Cryptococcus neoformans and Mycobacterium tuberculosis (30 and 27 patients, respectively). HIV-positive patients were more likely than HIV-negative patients to have mycobacteremia (57/182 vs. 0/64, P<. 0001), fungemia (38/182 vs. 2/64, P<.001), or polymicrobial BSIs (19/182 vs. 0/64, P<.002). Clinical predictors of BSIs included HIV infection, chronic diarrhea, lymphadenopathy, or splenomegaly. Mortality was higher among patients with than those without BSIs (P<. 001). Cohort-based microbiologic studies are critically important to diagnose emerging pathogens and to develop algorithms for empirical treatment of BSIs in developing countries.