BACKGROUND: Pathological Demand Avoidance (PDA) is a term increasingly used in the United Kingdom to describe children who obsessively resist everyday demands, going to extreme lengths to avoid these. There is debate about its relationship with both autism spectrum disorder (ASD) and oppositional defiant disorder (ODD). Unlike ASD, children with PDA are said to use socially manipulative avoidance strategies; and unlike ODD, they resort to extreme, embarrassing or age-inappropriate behaviour. To date, there has been little research into PDA, and it remains contentious. Currently, there are no questionnaire instruments available to aid consistency in description. This study reports the development and preliminary validation of the ‘Extreme Demand Avoidance Questionnaire’ (EDA-Q), designed to quantify PDA traits based on parent-reported information. METHODS: The validation study involved data from 326 parents of children aged 5-17 allocated to six groups based on information reported by parents about received diagnoses and behavioural difficulties: (a) typically developing children (N = 102), (b) children with ASD without disruptive behaviour (N = 36), (c) children with ASD with disruptive behaviour (N = 48), (d) children for whom PDA was suspected by parents (irrespective of other diagnoses) (N = 67), (e) children who had, according to parents, been identified as having PDA by a health professional, irrespective of other diagnoses (N = 50), and (6) disruptive behaviour or behavioural problems without suspected/identified ASD or PDA (N = 23). RESULTS: Although the Strengths and Difficulties Questionnaire (SDQ) did not differentiate PDA from those with ASD plus disruptive behaviour; score on the EDA-Q was significantly higher in PDA than all comparison groups. ROC analysis indicated good sensitivity (.80) and specificity (.85). Across all case groups, females scored higher than males on the EDA-Q. Separate cut-off scores were identified for older and younger age-groups. CONCLUSIONS: Our findings highlight the potential utility of the EDA-Q to assist the identification of this unusual profile for future research.