The aims of this prospective study were to determine the prevalence, characteristics, and impact of breakthrough pain in children with cancer. Twenty-seven pediatric inpatients with cancer (aged 7-18 years) who had severe pain requiring treatment with opioids and who received care in the Oncology Unit at the Children's Hospital at Westmead, Sydney, Australia participated in this study. The children responded to a structured interview (Breakthrough Pain Questionnaire for Children), designed to characterize breakthrough pain in children. Measures of pain, anxiety, and depressed mood were completed. Fifty-seven percent of the children experienced one or more episodes of breakthrough pain during the preceding 24 hours, each episode lasting seconds to minutes, occurring 3-4 times/d, and most commonly characterized as "sharp" and "shooting" by the children. Younger children (7-12 years) had a significantly higher risk of experiencing breakthrough pain compared to teenagers. No statistical difference could be shown between children with and without breakthrough pain in regard to anxiety and depression. The most effective treatment of an episode of breakthrough pain was a patient-controlled analgesia opioid bolus dose. Further studies of breakthrough pain in children and more effective treatment strategies in this age group are necessary.