Objective: Attention-deficit/hyperactivity disorder (ADHD) is associated with conflicted parent-child relationships. The underlying mechanisms of this association are not yet fully understood. We investigated the cross-sectional and longitudinal relationships between externalizing psychopathology in children with ADHD, and expressed emotion (EE; warmth and criticism) and psychopathology in mothers. Method: In this 6-year follow-up study, 385 children with an ADHD combined subtype were included at baseline (mean, 11.5 years, 83.4% male), of which 285 children (74%) were available at follow-up (mean, 17.5 years, 83.5% male). At both time points, measures of child psychopathology (i.e., ADHD severity, oppositional, and conduct problems), maternal EE, and maternal psychopathology (i.e., ADHD and affective problems) were obtained. Results: EE was not significantly correlated over time. At baseline, we found a nominally negative association (p < .05) between maternal warmth and child ADHD severity. At follow-up, maternal criticism was significantly associated with child oppositional problems, and nominally with child conduct problems. Maternal warmth was nominally associated with child oppositional and conduct problems. These associations were independent of maternal psychopathology. No longitudinal associations were found between EE at baseline and child psychopathology at follow-up, or child psychopathology at baseline and EE at follow-up. Conclusions: The results support previous findings of cross-sectional associations between parental EE and child psychopathology. This, together with the finding that EE was not stable over 6 years, suggests that EE is a momentary state measure varying with contextual and developmental factors. EE does not appear to be a risk factor for later externalizing behavior in children with ADHD.