Objective: The object was to examine the prevalence of ADHD among preschoolers, analyzing comorbidity, and the association with socio-demographic factors. Method: We conducted a two-phase epidemiological study of 1,104 preschoolers aged 3 to 6 years in Catalonia, Spain. The Early Childhood Inventory–4 (ECI-4) was administered to parents and teachers. Children at risk of ADHD were assessed using open-ended face-to-face interviews and were observed in a school setting. ADHD diagnoses were based on Diagnostic and Statistical Manual of Mental Disorders (4th ed.; DSM-IV) criteria. Results: The prevalence of ADHD diagnosis was 5.4%. Male sex and first-born status were risk factors for ADHD. Parents reported more symptoms (12.9%) than teachers (8.7%). Behavioral problems (odds ratio [OR] = 12, p = .001), autism spectrum disorder problems (OR = 9.5, p = .001), and obsessive-compulsive problems and tics (OR = 5.9, p = .001) were specifically related to ADHD diagnosis. Mother’s health status and school achievement were lower in ADHD children. Conclusion: Even at early stages of development, ADHD has high rates of comorbidity and a significant impact on school performance and family health.