PURPOSE: The educational environment's characteristics (e.g., class size) may affect attention-deficit/hyperactivity disorder (ADHD) diagnosis. We investigated changes in the number of individuals newly diagnosed with ADHD according to regional differences in educational conditions in South Korea. METHODS: With increasing class sizes, we hypothesized that teachers may experience more difficulty managing their students with hyperactive/impulsive symptoms or become less sensitive in detecting students with inattentive symptoms, causing a change in referrals for ADHD assessment. The annual number of ADHD diagnoses for each region was obtained from the National Health Insurance Service. The effects of educational conditions (i.e., number of students per teaching staff member, students per class, and private classes per 10,000 population) on newly diagnosed ADHD cases were analyzed separately for young people and adults. RESULTS: In the final adjusted quasi-Poisson regression model without interactions, we observed a negative correlation between both the number of students per class and the number of private classes per 10,000 population and ADHD diagnosis in young people and adults. Further analysis revealed that a greater number of private classes in a region was associated with a decrease in ADHD diagnosis, especially in the lowest educated districts. By contrast, a higher number of students per class in kindergarten was associated with an increase in ADHD diagnosis, especially in the highest educated districts. CONCLUSION: The educational environment's regional characteristics may affect ADHD detection and diagnosis. This association's direction may vary according to the regional educational level. A different association may also be observed in younger children.