Paediatric day surgery is a potentially stressful and threatening experience for children and parents. The purposes of this study were to investigate the effects of paediatric day surgery on anxiety level of Hong Kong Chinese children and their parents, and to examine the relationship between children's preoperative anxiety and their level of cooperation during induction and postoperative periods. A total of 112 children admitted for circumcision with their parents in a day surgery unit were recruited. The state anxiety level for both children and parents was assessed at the time of admission and again at 4 hours after operation. Children's level of cooperation during the induction and the postoperative periods were also assessed. The results showed that the preoperative anxiety level for both children and their parents was relatively high. Children with high preoperative anxiety levels were associated with lower levels of cooperation during induction and postoperative periods. The results also indicated that parents' high state anxiety was associated with their children's high state anxiety. This study designated that preoperative intervention to minimize children and parents' anxiety is crucial and that there is still room for improvement.