Objective: Family environmental factors as well as parental attention-deficit/hyperactivity disorder (ADHD) status have shown associations with variability in ADHD. The purpose of the present study was to examine the links among family environment, parental psychiatric diagnosis, and child impairment within a sample of ADHD-affected sibling pairs (ASPs) ages 5 to 18 years. Method: Parents in 220 ASP families completed a measure of family functioning, the Family Environment Scale. Children's impairment was measured by clinical ratings of global functioning and by maternal ratings of behavior. Results: Parents of children with ADHD rate their families as higher in conflict and lower in achievement and organization than normative samples. High family conflict is significantly associated with impairment in ADHD ASPs accounting for approximately 40% of the sibling similarity in impairment. Parental psychiatric diagnosis revealed no significant direct link to sibling impairment, but rather a significant indirect link to impairment mediated by family conflict. Direct associations with parental diagnosis depend on birth order of the ASP members despite the comparable mean impairment scores for older and younger ADHD siblings. Conclusions: There are strong links between impairment in children with ADHD and family environment. Different processes and mechanisms may contribute to impairment in different children in the same family. (Contains 3 figures and 2 tables.)