When a child is diagnosed with cancer, parents try to understand why the cancer developed. Although usually it is not possible to explain what caused an individual child's cancer, clinical experience has shown that parents do form theories about the origins of their child's illness although, or perhaps because, no one knows the actual cause. A parent-completed epidemiology questionnaire (EQ), designed to provide a comprehensive and general epidemiology data base for studies conducted by the Childrens Cancer Group, included an open-ended item ("Do you have any additional comments or concerns about anything that could have caused or contributed to your child's illness?"). A convenience sample of 500 EQs containing responses to the open-ended question was reviewed independently by two experienced pediatric oncology nurses. Statements contained in the responses were categorized into 12 major themes according to content: concern about environmental exposures (n = 303), concern about family health history (n = 270), specific causality attribution (n = 39), puzzlement (n = 24), concern with cancer "clusters" (n = 23), concern with stress (n = 22), altruism (n = 15), specific feedback requests (n = 11), myths/misconceptions (n = 5), advocation of preventive education/screening (n = 4), active information-seeking (n = 6), and parental self-blame (n = 4). These themes or concerns provide useful information that can be applied in planning educational and supportive clinical interventions, as well as further research.