Few studies have examined the epidemiology of preschoolers' psychopathology. This study included 796 4-year-old children recruited from schools and pediatric practices in a diverse, urban area. Psychiatric disorder was assessed by a structured interview adapted for preschool children and by questionnaire. The most common disorders were oppositional defiant disorder (ODD) and attention deficit hyperactivity disorder (ADHD). Generalized anxiety disorder (GAD) and depressive disorders were reported in less than 1% of the sample. Race/ethnicity differences were not significant. Gender differences showed ADHD-inattentive type more common among boys, with no gender differences for GAD, major depressive disorder, dysthymia, separation anxiety disorder, or ODD at any level of impairment. The overall comorbidity rate was 6.4%. Approximately 3% of individuals receiving a diagnosis had received mental health services. (Contains 3 tables.)