The incidence of bacterial infections in general and of bacteremia in particular is high among patients with acquired immunodeficiency syndrome (AIDS). The factors influencing the prognosis of bacteremia in these patients are not well known. In order to better define those factors associated with a poor prognosis, all episodes of bacteremia or fungemia in patients with AIDS who were hospitalized in four general hospitals between 1 September 1987 and 31 December 1996 were studied prospectively. Among 1,390 patients diagnosed with AIDS, 238 (17.1%) developed 274 episodes of bacteremia or fungemia. Mortality related to bacteremia was 21.3%. Variables associated with high mortality were fungemia (odds ratio [OR], 6.19; 95% confidence interval [CI], 1.99 - 19.28), hypotension (OR, 19.65; 95%CI, 7.42 - 52.07), inappropriate antimicrobial treatment (OR, 16.94; 95%CI, 4.92 - 58.32), and unknown origin of bacteremia (OR, 3.93; 95%CI, 1.58 - 9.76). The mortality rate among patients with at least one of these factors was 46.7%, whereas in patients without any of these factors, the rate was 4.9% ( P < 0.001). Bacteremic episodes of unknown origin were significantly more frequently associated with community acquisition ( P = 0.001), inappropriate antimicrobial treatment ( P = 0.04), and etiology by gram-negative microorganisms or fungi ( P < 0.001) and were significantly less frequently associated with the presence of a previous intravenous catheter ( P = 0.004), resulting in peculiar etiologic and epidemiological profiles. The factors that influence the outcome of AIDS patients who develop bacteremia are sometimes avoidable or known during the first days after admission. Therefore, knowledge about these factors could improve the prognosis of bloodstream infections in this population.