Measurement of pediatric cancer patients' health-related quality of life (HRQL) in phase III randomized, controlled clinical trials is being recognized increasingly as an essential component in evaluating the comprehensive health outcomes of modern anti-neoplastic treatment protocols. Use of a brief core measure of HRQL plus disease-specific symptom modules is a way to assess specific HRQL outcomes with a minimum of subject burden. Demonstrating a measure's feasibility, reliability and validity also represents children's ability to provide reliable and valid responses to HRQL questions. The Pediatric Cancer Quality of Life Inventory (PCQL) Modular Approach consists of a 15-item core measure of HRQL and 2 specific symptom modules: pain and nausea. To validate a patient-report form and a parentreport form, the PCQL was administered to 291 pediatric cancer patients and to their parents. Feasibility and range of measurement, as well as patient-parent concordance, were assessed. Internal consistency reliability was assessed via Cronbach's alpha. Validity was determined by the knowngroups approach and by correlating PCQL scores with days missed from school. Patients had minimal missing data, and the range of measurement for the items was good. Patientparent concordance was large but not perfect. For both patient and parent forms, internal consistency reliability of the PCQL core scale (0.83 and 0.86, respectively) was strong. The internal consistency reliabilities of the 2 symptom modules for both patient and parent forms were in the acceptable range for group comparisons. Regarding clinical validity, the core scale and the 2 symptom modules distinguished between patients on and off treatment for both patient and parent reports. Further, both patient and parent reports correlated with days of missed school in the past 6 and 12 months. The PCQL Modular Approach has demonstrated acceptable internal consistency reliability and clinical validity for both patient-report and parent-report forms. By implication, children are capable of providing reliable and valid responses to these HRQL questions.