In Pediatrics, LDI Senior Fellow Alexander Fiks and colleagues evaluate the change in the diagnosis of attention-deficit/hyperactivity disorder (ADHD) and in the prescribing of stimulants to children 4 to 5 years old after release of the 2011 American Academy of Pediatrics guideline. The authors use electronic health record data extracted from 63 primary care practices, including preventive visits from children 48 to 72 months old receiving care from January 2008 to July 2014, to compare rates of ADHD diagnosis and stimulant prescribing before and after guideline release. Among 87,067 children with 118,957 visits before the 2011 guideline and 56,814 with 92,601 visits after the guideline, children received an ADHD diagnosis at 0.7% of visits before and 0.9% after guideline release, and had stimulant prescriptions at 0.4% of visits in both periods. A significantly increasing pre-guideline trend in ADHD diagnosis ended after guideline release, and the rate of stimulant medication use remained constant before and after guideline release. Patterns of change from before to after the guideline varied significantly across practices. In conclusion, the release of the 2011 guideline that addressed ADHD in preschoolers was associated with the end of an increasing rate of diagnosis, and the rate of prescribing stimulants remained constant. These are reassuring results given that a standardized approach to diagnosis was recommended, and stimulant treatment is not the first-line therapy for this age group.