In 2009 the American College of Emergency Physicians, American Academy of Pediatrics, and Emergency Nurses Association jointly published guidelines for pediatric readiness for all emergency departments (EDs). In 2012-2013, the National Pediatric Readiness Project completed a Web-based assessment of EDs, generated a weighted pediatric readiness score (WPRS) based on compliance with the published guidelines, established a baseline of pediatric readiness in the United States, and provided individual EDs with a roadmap for improvement. Our goals were to engage a large health care system, to assess the effect of multi-faceted interventions aimed at improving pediatric readiness, and to perform a qualitative evaluation to identify components of the interventions that were thought to be most influential. We performed a mixed methods study using a “before-after” framework among 19 EDs in the Kaiser Permanente Northern California (KPNC) health care system. Each ED was assessed at baseline (in 2012) using the WPRS, a scale ranging from 0 - 100, then re-assessed (in 2015) following various ED-specific and regional interventions to improve readiness. Semi-structured interviews were then performed with emergency physician leaders from each institution. Interviews were recorded, transcribed, and examined for themes using a structured qualitative analysis. Nineteen EDs completed both the 2012 and 2015 assessments with a median WPRS of 70.0 in 2012 and a median WPRS of 77.8 in 2015 (difference=+7.8, p=0.09). Of the 19 EDs, 12 (63%) improved > 5 points, 4 (21%) EDs declined > 5 points, and 3 (16%) remained within a 5 point range. Mean improvement was +7.7 points (95% CI -1.3, 16.7). Mean changes in WPRS by domain of pediatric readiness were: policies and procedures + 4.4 (95% CI 1.4, 7.4); quality improvement activities + 2.3 (95% CI -0.1, 4.6); pediatric emergency care coordinators +2.0 (95% CI -3.2, 7.2); equipment + 0.7 (95% CI -0.7, 2.1); staffing and training -0.5 (95% CI -2.1, 1.1); and patient safety initiatives - 1.2 (95% CI -2.1, -0.2). Semi-structured interviews were conducted with physician leaders from 9 of the 19 EDs, 6 with increased WPRS, 2 with decreased WPRS, and 2 with no change in WPRS. Eight out of nine physician leaders reported the WPRS was an accurate reflection of their ED’s pediatric readiness. Common themes from the interviews included: (1) the national survey in 2012 brought attention and administrative buy-in to the issue of pediatric readiness; (2) many pediatric readiness improvement activities were initiated by ED nursing; and (3) both an ED pediatric champion and collaboration with inpatient pediatrics were key to improvement. Examples of activities that physician leaders reported improved the care of children between the 2012 and 2015 assessments included: pediatric simulation, use of critical care telemedicine, formalizing transfer agreements, equipment checks, disaster drills, and the use of a weight-based medication dosing system. Weighted pediatric readiness scores improved in the majority of KPNC EDs between 2012 and 2015. The attention brought to pediatric readiness by the 2012 assessment, administrative buy-in, a pediatric champion in the ED, and collaboration with inpatient pediatrics were identified by emergency physician leaders as keys to improvement.