Objective: This study aims to determine the prevalence of Attention Deficit Hyperactivity Disorder (ADHD), considering both parents' and teachers' reports and clinical diagnosis, and to examine the relationship between ADHD prevalence and measurement tools, sex, age, and clinical presentations. Method: This two-phase epidemiological study was conducted on 689 preschoolers aged 3 to 6 years in Kocaeli, Turkey. In the first phase, the Conners Rating Scale--Revised Long Form and the ADHD Rating Scale--IV Preschool Version were administered to teachers after parents. In the second phase, diagnostic evaluation was performed using DSM-5 ADHD criteria for children scoring above the 93rd percentile on any scale, according to either parent or teacher reports. Results: The prevalence of ADHD was estimated to range from 2% to 12.6%, depending on the measurement method. The prevalence of ADHD diagnosis was 3.3%, with a male-to-female ratio of 1.9:1. The hyperactive-impulsive presentation (1.62%) was more common than the combined presentation (1.32%) and the inattentive presentation (0.44%). Conclusion: Our findings are consistent with the literature, emphasizing the importance of ADHD screening in preschool children for early diagnosis. Measurement tools used in ADHD screening and parent-teacher responses should be evaluated carefully.