The optimum septic shock vasopressor support strategy is currently debated. This study was performed to evaluate the efficacy and safety of norepinephrine (NE) and dopamine (DA) as the initial vasopressor in septic shock patients who were managed with a specific treatment protocol. A prospective, randomized, open-label, clinical trial was used in a medical intensive care unit comparing DA with NE as the initial vasopressor in fluid-resuscitated 252 adult patients with septic shock. If the maximum dose of the initial vasopressor was unable to maintain the hemodynamic goal, then fixed-dose vasopressin was added to each regimen. If additional vasopressor support was needed to achieve the hemodynamic goal, then phenylephrine was added. The primary efficacy end point was all-cause 28-day mortality. Secondary end points included organ dysfunction, hospital and intensive care unit length of stay, and safety (primarily occurrence of arrhythmias). The 28-day mortality rate was 50% (67/134) with DA as the initial vasopressor compared with 43% (51/118) for NE treatment (P = 0.282). There was a significantly greater incidence of sinus tachycardia with DA (24.6%; 33/134) than NE (5.9%; 7/118) and arrhythmias noted with DA treatment (19.4%; 26/134) compared with NE treatment (3.4%; 4/118; P < 0.0001), respectively. Logistic regression analysis identified Acute Physiologic and Chronic Health Evaluation II score (P < 0.0001) and arrhythmia (P < 0.015) as significant predictors of outcome. In this protocol-directed vasopressor support strategy for septic shock, DA and NE were equally effective as initial agents as judged by 28-day mortality rates. However, there were significantly more cardiac arrhythmias with DA treatment. Patients receiving DA should be monitored for the development of cardiac arrhythmias (NCT00604019).