Attention deficit/hyperactivity behaviors in children are often unnoticed until they reach school age; however, evidence suggests that attention deficit hyperactivity disorder (ADHD) tends to develop at an earlier age. Recent studies also indicate that perinatal maternal mental health is associated with ADHD symptoms in offspring. This study examines the association between maternal postpartum depression, early hyperactive/aggressive behavior, and later ADHD symptoms. This longitudinal cohort study used data from health checkups and surveys. Mothers with a postpartum Edinburgh postnatal depression scale score [greater than or slanted equal to] 9 were considered as having postpartum depression symptoms. The presence of early hyperactive/aggressive behavior was determined based on 3-year-old children's health checkup data. The ADHD rating scale (ADHD-RS) answered by the caregivers of 182 sixth-grade children was used for the analysis. ADHD-RS scores were significantly higher in children with early hyperactive and/or aggressive behavior (p = 0.002) and for children of mothers who had postpartum depression symptoms (p < 0.001). Early hyperactive/aggressive behavior did not mediate the predictive association of maternal postpartum depression symptoms with ADHD symptoms in sixth grade. ADHD symptoms in the sixth grade were independently associated with childhood hyperactive/aggressive behavior at age 3 and maternal postpartum depression symptoms.