The present study examined trajectories of individual Diagnostic and Statistical Manual of Mental Disorders (4th ed.) symptoms of attention-deficit/hyperactivity disorder (ADHD) and oppositional defiant disorder (ODD) across the preschool years in children with ADHD. It also evaluated whether preschool symptoms vary in their ability to discriminate children who later meet criteria for ADHD from typically developing children. ADHD and ODD symptoms were assessed annually in 75 ethnically diverse children (46 boys) who presented with behavior problems at age 3 and met criteria for ADHD 3 years later, and in 51 typically developing children (26 boys). Children with ADHD generally exhibited stable levels of hyperactivity/impulsivity but increases in several symptoms of inattention. Most ADHD symptoms showed at least fair utility in discriminating children with and without ADHD; however, 3 symptoms of inattention (carelessness, losing things, and forgetfulness) and 1 symptom of hyperactivity/impulsivity (blurting out answers) had relatively poor utility. These symptoms demonstrated only somewhat greater utility at age 4, but by the age of 5 were better able to classify children. Children with ADHD exhibited increases in several ODD symptoms, including symptoms related to negative affect. Although most symptoms of hyperactivity/impulsivity appear to extend well down to age 3, more developmentally appropriate symptoms of inattention may be required to develop more sensitive assessments for 3- and 4-year-old children.