This paper reports on the results of two studies conducted to further validate the Abu-Saad Paediatric Pain Assessment Tool, a Dutch-language questionnaire developed to assess pain in school-age children. Children of 5 to 15 years of age reported in the first study their post-operative pain before and after the administration of analgesic medication using word descriptors of pain, the 10-cm scale, the Oucher, and a visual analogue scale (VAS). Lower mean pain scores at 4-hourly intervals post-analgesia supported the construct validity of the tool. Convergent validity was supported in both studies when scores on all concurrently administered measures positively correlated with the pain tool. In the second study, the correlations between pain and fear, a concurrently used measure with the pain instruments, were low, substantiating the discriminant validity of the pain tool. The significance of the results in relation to instrument development and multidimensional pain assessment in children are further discussed and elaborated.