OBJECTIVE We sought to measure trends in evaluation and management of children with simple febrile seizures (SFSs) before and after the American Academy of Pediatrics updated guidelines published in 2011. METHODS In this retrospective, cross-sectional analysis, we used the Pediatric Health Information System database comprising 49 tertiary care pediatric hospitals in the United States from 2005 to 2019. We included children aged 6 to 60 months with an emergency department visit for first SFS identified using codes from the International Classification of Diseases, Ninth Revision, and International Classification of Diseases 10th Revision. RESULTS We identified 142 121 children (median age 21 months, 42.4% female) with an emergency department visit for SFS. A total of 49 668 (35.0%) children presented before and 92 453 (65.1%) after the guideline. The rate of lumbar puncture for all ages declined from 11.6% (95% confidence interval [CI], 10.8% to 12.4%) in 2005 to 0.6% (95% CI, 0.5% to 0.8%) in 2019 (P < .001). Similar reductions were noted in rates of head computed tomography (10.6% to 1.6%; P < .001), complete blood cell count (38.8% to 10.9%; P < .001), hospital admission (19.2% to 5.2%; P < .001), and mean costs ($1523 to $601; P < .001). Reductions in all outcomes began before, and continued after, the publication of the American Academy of Pediatrics guideline. There was no significant change in delayed diagnosis of bacterial meningitis (preperiod 2 of 49 668 [0.0040%; 95% CI, 0.00049% to 0.015%], postperiod 3 of 92 453 [0.0032%; 95% CI, 0.00066% to 0.0094%]; P = .99). CONCLUSIONS Diagnostic testing, hospital admission, and costs decreased over the study period, without a concomitant increase in delayed diagnosis of bacterial meningitis. These data suggest most children with SFSs can be safely managed without lumber puncture or other diagnostic testing.