Objective: To quantify the effect of intra-hospital patient flow on Emergency Department (ED) performance targets and indicate if the expectations set by the NHS England five year forward review are realistic in returning emergency services to previous performance levels. Design: Linear regression analysis of routinely reported trust activity and performance data using a series of cross-sectional studies. Setting: NHS trusts in England submitting routine nationally reported measures to NHS England. Participants: 142 acute non-specialist trusts operating in England between 2012 and 2016. Main outcome measures: The primary outcome measures were: proportion of four-hour waiting time breaches and cancelled elective operations. Methods: Univariate and multivariate linear regression models were used to show relationships between the outcome measures, and various measures of trust activity including: empty day-beds, empty night-beds, day-to-night bed ratio, ED conversion ratio and delayed transfers of care. Results: Univariate regression results using the outcome of four-hour breaches showed clear relationships with: empty night-beds and ED conversion ratio between 2012-2016. The day-to-night bed ratio showed an increasing ability to explain variation in performance between 2015-2016. Delayed transfers of care showed little evidence of an association. Multivariate model results indicated that the ability of patient flow variables to explain four-hour target performance had reduced between 2012-2016 (19% to 12%), and had increased in explaining cancelled elective operations (7% to 17%). Conclusions: The flow of patients through trusts is shown to influence ED performance, however performance has become less explainable by intra-trust patient flow between 2012 and 2016. Some commonly stated explanatory factors such as delayed transfers of care showed limited evidence of being related. The results indicate some of the measures proposed by NHS England to reduce pressure on EDs may not have the desired impact on returning services to previous performance levels.