Summary Objective To explore the reasons why nursing staff use the subjective “worried” Medical Emergency Team (MET) calling criterion and compare the outcomes of calls activated using the “worried” criterion with those calls activated using “objective” criteria such as vital sign abnormalities. Methods A descriptive study of MET calls in six acute hospitals over a 12 months period. Outcomes for “objective” and “worried” calls were compared. Results The “worried” criterion was used to activate 29% of 3194 MET calls studied; it was the single most common reason for a MET call. Half (51.7%) of the “worried” calls were related to problems with Airway, Breathing, Circulation or Neurology. ‘Breathing’ problems accounted for the largest proportion (35.2%). A low oxygen saturation by pulse oximetry (SpO2 ) ( n = 249, 26.9%) and ‘respiratory distress’ ( n = 133, 14.4%) were the most common reasons for a “worried” call. Only 1.1% (10) of calls triggered by the “worried” criteria had cardiac arrest as an outcome compared with 170 calls (7.6%) for “objective” criteria. The proportion of patients who remained in a general ward area after MET calls was higher for the “worried” calls. Conclusions The “worried” criterion was the most frequent reason for MET calls, implying a high degree of empowerment and independent action by nursing staff. Low SpO2 and respiratory distress were the most common causes for concern. There was a significant difference between MET calls triggered by “worried” criteria and “objective” criteria for outcomes immediately following MET ( p < 0.001). Further assessment and refinement of MET triggers particularly in relation to respiratory distress and pulse oximetry may be needed.