The present study examined the bidirectional relation between multiple measures of family functioning and attention-deficit/hyperactivity disorder (ADHD) symptoms across the preschool years. Additionally, it examined if these relations held when comorbid child ODD symptoms and parental ADHD symptoms were taken into account. Participants included 258 (138 boys) 3-year-old children (M = 44.13 months, SD = 3.39) with and without behavior problems and their parents who took part in a 3-year longitudinal study. Evidence was found for both parent and child effects, depending on the measure of family functioning. Specifically, maternal depressive symptoms, overreactive parenting, and stressful life events were predictive of child hyperactivity/impulsivity (HI). Additionally, child HI was predictive of maternal depressive symptoms, warm parenting, and stressful life events. For all measures of family functioning, observed parent and/or child effects held when comorbid child ODD symptoms and parental ADHD symptoms were taken into account. This suggests that while child ODD and parent ADHD symptoms are related to family functioning, they do not fully explain the relation between child hyperactivity/impulsivity and family functioning. The relations between child inattention and family functioning were mostly consistent with those for child HI, but child HI tended to have somewhat more consistent and stronger relations with family functioning than child inattention. These findings suggest that targeting child hyperactivity/impulsivity and inattention symptoms, maternal depressive symptoms, maternal parenting practices, and stressful life events each hold promise for attenuating the negative mutual influence of child ADHD symptoms and family functioning over time.