Sibling and friendship relationships are quite similar, in that they are both dyadic and reciprocal, and offer the child feelings of warmth, trust, loyalty and intimacy. Children with Attention Deficit/Hyperactivity Disorder (ADHD) often experience difficulties in their interpersonal relationships. The aim of this study was to evaluate the quality of and to acknowledge possible links between sibling and friendship relationships of children with ADHD and typical development. Participants were 40 ADHD children and 120 typically developing peers, attending Greek primary schools (M[subscript age] = 9.51 years, SD = 1.1). The Sibling Relationship Questionnaire, sociometric nominations, and the Friendship Quality Questionnaire were used. The results showed that children with ADHD had greater antagonism with their siblings and fewer reciprocated friendships. Significant differences in friendship quality between the two groups were observed. Sibling relationships were found to influence all children's number of mutual friends, and the friendship quality of the typically developing children.