A prevalence-based cost-of-illness study using a societal perspective was conducted to investigate the cost-of-illness in clinically anxious youth aged 8-18 in The Netherlands. Discriminant validity of the cost diary used was obtained by comparing costs of families with an anxious child (n = 118) to costs of families from the general population (n = 41). To examine the convergent validity, bottom-up acquired costs derived from cost diaries were compared to top-down acquired costs obtained from national registrations. Bottom-up acquired costs measured by means of cost diaries amounted to Euro 2,748 per family of a clinically referred anxious child per annum. Societal costs of families with clinically anxious children were almost 21 times as high compared to families from the general population. With respect to convergent validity, total health care costs using the bottom-up approach from clinically anxious children were quite comparable to those of top-down data of anxious children, although costs within the subcategories differed considerably. Clinical anxiety disorders in childhood cost the Dutch society more than 20 million euros a year. Based on results of discriminate and convergent validity, the cost diary seems a valid method in establishing cost-of-illness in childhood anxiety disorders.