Attention deficit/hyperactivity disorder (ADHD) is a developmental disorder that may have a chronic and pervasive impact on the child's function and cause long-term stress to parents. A higher rate of depression is associated with mothers of children with ADHD. This observational study aimed to investigate the effect of maternal depression and the child's ADHD on the quality of the parent-child interaction in children with ADHD and their mothers with depression. The study participants comprised 39 mother-son dyads including children with ADHD and mothers with depression, children with ADHD and mothers without depression, and children without ADHD and mothers without depression. The Specific Affect Coding System, 20-code version was used to code interactional affect, including positive engagement, negative engagement, negative disengagement, and neural affect. There were no statistically significant group-by-context interaction effects or group effects on all affective variables between the group of children with ADHD and mothers without depression and the group of children without ADHD and mothers without depression. Stimulant medication may account for these nonsignificant findings. No significant difference of positive affect between neutral and conflict-solving contexts was observed in depressed mothers whose children were diagnosed as ADHD. Children with ADHD whose mothers were depressed were less positive in their parent-child interaction compared with children in the other groups. Maternal depression may play an important role in the affective presentation of dyads of children with ADHD and mothers with depression. Implications for clinical practice and future research are provided. (Contains 4 tables and 2 figures.)