We attempt to explain the co-variation between ADHD and symptoms of depressed mood, focusing on the family context and testing whether the mother-child relationship mediates or moderates the link between child ADHD and youth depressed mood symptoms. In a longitudinal study, we used mother and youth reports for 596 Swedish youth, 50% boys, from a community sample at 10, 15, and 18 years of age. The results did not support the mediation hypothesis. Only one moderation effect was found. Mother-child conflicts in mid-adolescence, as rated by mothers, increased symptoms of depressed mood symptoms in late adolescent only for youth with high levels of hyperactivity symptoms. However, depressed mood symptoms at age 18 were predicted by low mother-child involvement in mid-adolescence, over and above the effects of inattention symptoms. This latter finding was consistent across mother and youth ratings of the relationship. Implications of these results are discussed.