Crowding in emergency departments (EDs) is increasing in many health systems. Previous studies of the relationship between crowding and care quality are limited by the use of data from single hospitals, a focus on particular patient groups, a focus on a narrow set of quality measures, and use of crowding measures which induce bias from unobserved hospital and patient characteristics. Using data from 139 hospitals covering all major EDss in England, we measure crowding using quasi-exogenous variation in the volume of EDs attendances and examine its impacts on indicators of performance across the entire EDs care pathway. We exploit variations from expected volume estimated using high-dimensional fixed effects capturing hospital-specific variation in attendances by combinations of month and hour-of-the-week. Unexpected increases in attendance volume result in substantially longer waiting times, lower quantity and complexity of care, more patients choosing to leave without treatment, changes in referral and discharge decisions, but only small increases in reattendances and no increase in mortality. Causal bounds under potential omitted variable bias are narrow and exclude zero for the majority of outcomes. Results suggest that physician and patient responses may largely mitigate the impacts of demand increases on patient outcomes in the short-run.