The impact of "goal-directed" abdominal ultrasound (US) on real-time decision making in the emergency department (ED) was studied, with specific emphasis on the certainty of diagnosis, treatment, and disposition plans. A prospective, interventional study enrolled 212 patients at a county teaching hospital ED, who underwent bedside US by experienced ED sonographers. A study questionnaire was completed documenting the US indication, working diagnosis, treatment, and disposition plan. The physicians assigned pre-test and post-test levels of certainty for the diagnosis, treatment plan, and disposition on an integral scale from 1 to 10. Scores for diagnosis were further categorized into low (1-3), moderate (4-7) and high certainty of disease. Absolute mean changes in level of certainty for diagnosis, treatment, and disposition were 3.2 (95% CI 3.1-3.3), 2.0 (95% CI 1.9-2.1), and 1.9 (95% CI 1.8-2.0), respectively. The direction of change after US for certainty of diagnosis was evenly split, with 47% increasing and 47% decreasing. The majority of patients categorized as either high or low certainty of disease had US results concordant with the physician's initial assessment. However, 16% moved from either high to low or from low to high certainty categories after US. Patients with moderate certainty moved evenly to either the low or high post-test category in 97% of cases. Treatment and disposition decisions were less impacted by US, with the majority of cases increasing in certainty irrespective of the US results. Bedside ultrasonography in the ED has an important impact on real-time decision-making, particularly in terms of the certainty of diagnosis.