The objective was to evaluate the effect of mandated nurse-patient ratios (NPRs) on emergency department (ED) patient flow.Two institutions implemented an electronic tracking system embedded within the electronic medical record (EMR) of two EDs (an academic urban, teaching medical center-Hospital A; and a suburban community hospital-Hospital B), with a combined census of 60,000/year, to monitor real-time NPRs and patient acuity, such that compliance with state-mandated ratios could be prospectively monitored. Data were queried for a 1-year period after implementation and included patient wait times (WTs), ED care time (EDCT), patient acuity, ED census, and NPR status for each nurse, patient, and the ED overall. Median WT and EDCT with interquartile ranges (IQRs) were analyzed to determine the effect of NPR status of each patient, nurse, and the ED overall. To control for factors that could affect the "within the mandated ratio" and the "outside of the mandated ratio" status, including patient volume and acuity, log-linear regression models were used controlling for specified factors for each hospital facility and combined.There were a total of 30,404 (50.9%) patients who waited in the waiting room prior to being placed in an ED bed (53.8% at Hospital A and 46.4% at Hospital B). Patients who waited at Hospital A waited a median duration of 55 minutes (IQR = 15-128 minutes), compared with 32 minutes (IQR = 12-67 minutes) at Hospital B with a combined median WT of 44 minutes (IQR = 13-101 minutes). In the log-linear regression analysis, WTs were 17% (95% confidence interval [CI] = 10% to 25%, p < 0.001) longer at Hospital A and 13% (95% CI = 3% to 24%, p = 0.008) longer at Hospital B (combined 16% [95% CI = 10% to 22%, p < 0.001] longer at both sites) when the ED overall was out-of-ratio compared to in-ratio. There were a total of 45,660 patients discharged from both EDs during the study period, from which EDCT data were collected (26,894 in Hospital A and 18,766 in Hospital B). Median EDCT was 184 minutes (IQR = 97-311 minutes) at Hospital A, compared to 120 minutes (IQR = 63-208 minutes) at Hospital B, for a combined median EDCT of 153 minutes (IQR = 81-269 minutes). In the log-linear regression analysis, the EDCT for patients whose nurse was out-of-ratio were 34% (95% CI = 30% to 38%, p < 0.001) longer at Hospital A and 42% (95% CI = 37% to 48%, p < 0.001) longer at Hospital B (combined 37% [95% CI = 34% to 41%, p < 0.001] longer at both sites) when compared to patients whose nurse was in-ratio.In these two EDs, throughput measures of WT and EDCT were shorter when the ED nurse staffing were within state-mandated levels, after controlling for ED census and patient acuity.