Background: Irritable mood has recently become a matter of intense scientific interest. Here, we present data from two samples, one from the United States and the other from the United Kingdom, demonstrating the clinical and research utility of the parent- and self-report forms of the Affective Reactivity Index (ARI), a concise dimensional measure of irritability. Methods: The US sample (n = 218) consisted of children and adolescents recruited at the National Institute of Mental Health meeting criteria for bipolar disorder (BD, n = 39), severe mood dysregulation (SMD, n = 67), children at family risk for BD (n = 35), or were healthy volunteers (n = 77). The UK sample (n = 88) was comprised of children from a generic mental health setting and healthy volunteers from primary and secondary schools. Results: Parent- and self-report scales of the ARI showed excellent internal consistencies and formed a single factor in the two samples. In the US sample, the ARI showed a gradation with irritability significantly increasing from healthy volunteers through to SMD. Irritability was significantly higher in SMD than in BD by parent-report, but this did not reach significance by self-report. In the UK sample, parent-rated irritability was differentially related to emotional problems. Conclusions: Irritability can be measured using a concise instrument both in a highly specialized US, as well as a general UK child mental health setting. (Contains 4 tables, 1 footnote, and 1 figure.)