Valid studies comparing the clinical characteristics among adult, elderly, and the oldest old bacteremic patients are lacking. We conducted a prospective, observational study in the emergency department (ED) of a university medical center between June 2001 and June 2002. All patients >18 years of age who registered in the ED with a clinically significant, culture-positive, bloodstream infection (BSI) were enrolled. Patients were divided into 3 groups based on age: 1) oldest old (> or =85 yr), 2) elderly (65-84 yr), and 3) adult (18-64 yr). The clinical and laboratory manifestations and 30-day mortality were recorded. Group comparisons were performed using the chi-square test or analysis of variance (ANOVA) test, as indicated. Survival was analyzed using the Kaplan-Meier method and the Cox-regression model, adjusted for potential confounders.A total of 890 cases of community-acquired BSI were eligible for analysis. Compared to the adult group, both the elderly and the oldest old patients had more atypical clinical manifestations, a higher propensity to develop organ failure, and a worse prognosis. Elderly patients had significantly less tachycardia (p = 0.001), but more acute respiratory (p = 0.007) and renal failure (p = 0.037); the oldest old patients had more afebrile episodes (p = 0.006), leukocytosis (p = 0.012), and more patients developed respiratory failure (p = 0.009), acute renal failure (p = 0.011), septic shock (p = 0.022), and altered mental status (p = 0.013). Urinary tract infections were the main source of BSI for both the elderly and oldest old, while the oldest old patients had significantly more pneumonia than the elderly or adults. As a group, older patients had fewer signs and symptoms of BSI, but a higher risk of organ failure and a worse prognosis than younger patients.