Objective: The importance of the early recognition of shock in patients presenting to emergency departments is well recognized, but at present, there is no agreed practical definition for undifferentiated shock. The main aim of this study was to validate an a priori clinical definition of shock against 28-day mortality. Design, setting and subjects: This prospective, observational, cross-sectional, single-center study was conducted in Hong Kong, China. Data were collected between July 1, 2012, and January 31, 2013. An a priori definition of shock was designed, whereby patients admitted to the resuscitation room or high dependency area of the emergency department were divided into 1 of 3 groups - no shock, possible shock, and shock. The primary outcome was 28-day mortality. Secondary outcomes were in-hospital mortality or admission to the intensive or coronary care unit.Measurements and main results: A total of 111 patients (mean age, 67.2 ± 17.1 years; male = 69 [62%]) were recruited, of which 22 were classified as no shock, 54 as possible shock, and 35 as shock. Systolic blood pressure, mean arterial pressure, lactate, and base deficit correlated well with shock classifications (P <.05). Patients who had 3 or more positively defined shock variables had a 100% poor composite outcome rate (5 of 5). Patients with 2 shock variables had a 66.7% (4 of 6) poor composite outcome rate.Conclusions: A simple, practical definition of undifferentiated shock has been proposed and validated in a group of patients presenting to an emergency department in Hong Kong. This definition needs further validation in a larger population and other settings.