Many computer simulation models of A&E departments have been developed to aid clinicians and managers to maintain and improve performance of their departments. Here we present a model that can be also used to understand changes in performance that may occur as a result of the 4-hour target regime in the English NHS. The model simulates the performance resulting from normal activity and the differences between this and actual performance are revealing. Results from 2 departments are presented to demonstrate this mode of model use. These reveal the extent of special action taken in some A&E departments as patients approach the target time and also show the true, underlying performance of the departments.