Background: Functional impairment is needed to make an attention deficit hyperactivity disorder (ADHD) diagnosis, but there is a paucity of instruments addressing this issue. Objective: Perform psychometric analysis of a functional impairment scale (FIE). Methods: A sample of 320 individuals, including ADHD probands, their siblings and parents, filled the FIE. We analyzed psychometric properties for the entire sample and age groups. Factor structure was determined by a principal component factor analysis, using oblique rotation with Kaiser normalization and Eigenvalues higher than 1. Cronbach's alpha and SpearmanBrown were calculated. Results: Family analysis revealed four components: a) "family life", b) "self-perception", c) "performance" and d) "social life". Adults' analysis revealed two components: a) "family life, social life and self-perception" and b) "performance". Children showed the domains: a) "performance and social life", b) "self-perception" and c) "family life" components. Cronbach's alpha were above 0.9 in all components. Discussion: Results revealed up to four domains depending on the group considered. Different life demands might explain the variability of domains on the groups.